“The thing that drives me crazy about your country,” says João Magueijo, polishing off another razor clam, “is the showers. I love cold showers. But this doesn’t exist in the UK. It’s been a source of irritation for years. Just warm or hot! What if you don’t like it? What are you supposed to do?”

He fishes his phone from his pocket and brings up a picture of a sign he saw on a visit to a university gym recently. It is a diagram of a communal shower, showing the temperature at which each unit is set. “Each shower is a prefixed temperature!” he says incredulously, his Portuguese accent thickening slightly as the excitement overtakes him. “There’s no cold water in sports centres!” As I’m trying to think of something to say, he jabs the phone at me again, a man longing to be understood. “The temperature is fixed! You can’t adjust it!” he repeats. “You don’t see why this is weird?”

Magueijo is not finished. In fact, he is barely even started. And he does not sound much like a theoretical physicist, which is what he is. Until recently, he was best known for his studies in cosmology, undertaken in 25 years at Cambridge and Imperial College. Above all, he is renowned for his work towards demonstrating that the speed of light is variable. This week, though, that lofty achievement has been displaced, at least for a while, by something rather less cerebral: a short book explaining why we Brits are a bunch of sex-mad, pissed-up, overweight hooligans and snobs. At the launch party, the publisher told guests: “When Byron was in Portugal, he wrote some very uncomplimentary things. Well, it took 200 years, but the reply has arrived.”

The book, with the bracing title Undercooked Beef (Bifes Mal Passados), was written in Portuguese; it was intended, Magueijo protests, only for Portuguese eyes. But then some enterprising character alerted the Sunday Times to its existence, and the paper published a story listing some of Magueijo’s more trenchant criticisms. Perhaps it is appropriate then that he is talking not in his adopted homeland, nor in his original one but, because he is attending a conference there, in the safely neutral territory of Madrid.

According to Magueijo, the article pointed out, Britain is “one of the most rotten societies in Europe, possibly the world”. British homes are less clean than “my grandmother’s poultry cage”. Our diet is “deplorable” and fish and chips is a dish that “makes you want to wash it with detergent before eating”. The north is “hideous”, the class system the source of “reciprocal fear and hate”. And then there’s the drinking, and the shagging, illustrated through a number of eye-popping anecdotes involving projectile vomit (the author’s own, over the wife of a fellow of St John’s College, Cambridge) and casual blowjobs. “I never met such a group of animals,” Magueijo concludes. “The English are unrestrained wild beasts and totally out of control.” This is not, incidentally, his first controversial literary output: when he was 16, he was kicked out of school for writing a rude essay about the headmistress. You cannot help wondering what his colleagues at Imperial will make of it.

He complains bitterly that this is an incomplete account, leaving out vital qualifications and counterpoints and, as we shall see, he may have a point. The aftermath has been, Magueijo says, quite a ride. He has declined to be photographed, rather to my surprise – he is a good-looking, confident man, with a youthful zip that belies his 47 years, and plainly not a shrinking violet. “Well,” he says, “this was already a very tense thing, to do this interview. I’ve had a very difficult week. The level of the emails … you wake up and there are 100 new ones in your mailbox, and half of them are basically abuse. Threats of violence, or ‘Fuck you, wanker’, or ‘Go back where you belong’.”

This sounds horrible, and perhaps it makes Magueijo regret putting himself in the firing line. Not a bit of it, he says: he’s just giving us a taste of our own medicine. “A lot of those things are real. I’m not apologising for it. It was a joke, but it’s a fair one. The whole travelogue culture, about the poor English person who has horrible things happening to him – everything is foul, the food is horrible, people are trying to steal from you – it’s your culture. You are such easy targets. It’s the easiest book I’ve ever written. I wrote the first draft in three weeks.”

In any case, he adds, the idea that he’s an outsider is not exactly right. And perhaps his sense of this tells us something about national pride, and self-criticism, and what it really means to belong. “I’ve lived in your country for 25 years,” he points out. There’s a pause. “Our country. Let’s call it our country.”

As João Magueijo is British, so Lucy Pepper, a writer and illustrator who moved from the UK to a village near Lisbon in 1999, is Portuguese. All the same, she retains some affection for the old country, and she finds the academic’s version of it unconvincing. As a columnist for a Lisbon newspaper, whose work often takes aim at Portugal’s own foibles, she has something at stake. “I’m a fan of the book’s right to exist,” she says. “But I’m not a fan of the book. Like, he says the British class system is so much more rotten than the Portuguese one. In Portugal class is much simpler, but it’s just as divisive. I wrote, ‘What the fuck?’ after that paragraph.”

She is unimpressed, too, by the one-country publication strategy. “I knew he wasn’t going to put it out in England, which I think is extreme cowardice on his part. If you’re going to say awful things about a country, you should front up and do it in their language.”

She does agree with some bits, though, like the complaint about drinking – that British men “have to drink like sponges, eat like skeletons, and throw up everything at the end of the evening”. Pepper is sipping on a beer with Tom Davis, an expat entrepreneur, and Nelson Vassalo, a Portuguese graphic designer. They’re in Mercado da Ribeira, where half of what was the biggest market in Lisbon has been converted into an elegant food hall with a few dozen kiosks and drinks stands, hundreds of enthusiastic customers, and absolutely no trouble – a neat counterpoint to the fearful cliches of a British night out.

Nor, they all say, is trouble ever as likely a consequence of drinking in Lisbon as it is in London or Liverpool. “People here do it slowly but surely,” Davis says. “They drink steadily through the night, and they wind up happy-drunk, rather than falling-over-vomiting drunk.” Vassalo agrees: “The Brits who come to our country, to the Algarve, these kids are getting wasted like crazy.”. “I wouldn’t enjoy a night like those, man.” Even the new students I’ve seen earlier, merrily winding through town with drinks in hand, seem to be taking things gently.

Vassalo’s not read it himself, but the book is a big hit in Portugal, with 20,000 sales already, a significant number in a small country. This may seem strange to a British audience, which perceives no special connection between the two nations, but in Lisbon it makes sense. “It’s a really deep-rooted thing that we’re each other’s oldest allies,” Davis explains – a fact known by very few in the UK.

“We have a big focus in our history class on all that,” adds Vassalo. “The British kingdom comes along pretty early on.” Then there’s the knotty question of the impact of Anglophilia and -phobia, and how the two mingle and complicate each other. “A lot of people don’t like us,” says Pepper. “But they like the culture we come out with. They like Britpop. They like Fawlty Towers.”

Portuguese cultural exports to the UK, on the other hand, are not so big, unless you count Nando’s, which, delicious though it is, is not even really Portuguese anyway. So maybe the attention the book has garnered is partly the product of something like the little-brother syndrome that plagues so many Brits about America: better to be hated than be the object of benign, ignorant indifference. Or, as the woman picking up a copy from the prominent display in Bertrand, the world’s oldest bookshop, puts it: “You may not want to know about us, but we want to know about you!”

But let’s say, I propose in the market, that a British writer planned a similar hit on Portugal, an act of beefy revenge. What would they focus on? One idea seems to turn up often: the suggestion that social conservatism and the limited ambitions of a generation that was simply relieved not to be living under a rightwing nationalist dictatorship (that lasted until 1974) have left the country short on creativity. “I hear a lot of, ‘We don’t do that here’,” says Pepper, who is in her 40s. “People of my age and older, there’s no openness to novelty.” For his twenty- and thirtysomething peers, Vassalo says, that has changed: the economic crisis has forced innovation on anyone who wants to stay put. But he agrees about the generation above. “They get comfortable, enjoy the sun, whatever,” he says. “I do believe that for [Magueijo’s] generation it was extremely important to get out of here. We don’t need to, because life is hard enough for us.”

The evening breaks up. Some hours later, at about three in the morning, a raucous chant goes up from a group of those freshers outside my hotel in Lisbon’s absurdly lovely Rossio Square. I peer out of the window in time to see one of them cheerfully pursuing a car down the road before falling flat on her face. Perhaps we are not all that different, after all.

The funny thing is, João Magueijo agrees with his young countryman’s analysis. That is exactly why he was so infuriated by the coverage his book has received in the UK. “I’m not saying what’s there is unfair,” he goes on. “But it’s only half of it.

“The other half is that your country has an incredible creative vibe. Not just in science, but in music, in art, in poetry – and that comes from the bad things. Life is so miserable, you have to do something just not to commit suicide. You punch someone in the face, it’s the same thing that leads you to write a beautiful poem.”

I have never really thought of this, and I don’t know if I agree, but I suppose it just about makes sense – a notion that may be more obvious to an outsider than an old hand.
Although, as Magueijo and I have already discussed, he is not really an outsider any more. “No one got it,” he adds, “but in the book it says that I am a mongrel. After all these years, you aren’t one thing or another, you’re something in between.”

He first became aware of it when he got a cab outside Lisbon airport, and tried to climb into the driver’s seat. “Oh shit,” he thought. “I’ve changed.” These days, he finds foreigners’ inability to pick up the obscure signals of British nuance as infuriating as Basil Fawlty might. “Hints just don’t exist in Latin culture,” he moans. “Of course, in England, people think I’m very abrupt. But when I’m abroad, I drop hints, and no one picks them up.”

If this is all true, it seems an odd way to show your allegiance – writing a book in a foreign language that tells another country how horrible we are. Magueijo cocks his head and smiles ruefully. “Well,” he says, “when I was about to finish the book, I felt so bad about what I was doing that I actually joined the Royal Astronomical Society, and the Institute of Physics. It’s meant to be a big moment in your career, but I’d never bothered.” To Magueijo’s mind, his flair as a scientist is the product of his mongrel nature: native enthusiasm, and what he sees as a British willingness to look foolish. “I have this Portuguese exuberance when I do science ... but I am part of this tradition now.” It’s a funny thing. In the end, the moment that Magueijo gave up our secrets was the moment he realised he belonged.

• This article was amended on 22 September 2014 to correct João Magueijo’s occupation. He is a theoretical physicist, not a theoretical physician.