The day of his victory, the prime minister reached out to first-time Tories with the “incredible truth that we now speak as a One Nation Conservative party literally for everyone from Woking to Workington; from Kensington, I’m proud to say, to Clwyd South; from Surrey Heath to Sedgefield; from Wimbledon to Wolverhampton”.

From Bolsover, where Denis Skinner lost his seat, to Britannia Bay, where Johnson has reportedly arrived, on Mustique.

From Johnson’s vacant holiday home, Chequers, to – if we must sacrifice his stylish alliteration for dull, topographical precision – Villa Oceanus, overlooking Britannia Bay, as above. Since the villa is available to rent, literally for everyone, for £20,000 a week, we can, courtesy of its website, picture Mr Johnson poolside in his swimmers, reciting The Iliad to an enraptured host, Count Bismarck; outlining his “mission to work night and day” with the villa’s (four) attentive staff; discussing the environment with his girlfriend, the celebrated conservationist Carrie Symonds.

Although an online calculator suggests that the couple’s return trip – minus security – will generate emissions of around 16.5 tonnes of CO2, Johnsonian conservation, if Johnson senior is any guide, suggests that this unwelcome addition (the UK annual average, per capita, is around 5.5 tonnes), can be fully offset by profound personal indifference. Indeed, this trip may be the first real indication of Johnson’s meaning, when he promised, in his victory speech, “the most far-reaching environmental programme”. Who guessed it would extend to a tiny island in the Caribbean?

As for another pledge, to “rise to the challenge and to the level of expectations”, Johnson’s first holiday as PM must surely surpass all predictions. True, the alacrity with which he saved Zac – now Lord – Goldsmith from the will of the Richmond people confirmed suspicions about his undeviating flakiness, without being strictly unprecedented. The Blairs, alike in indifference to appearances, ennobled friends and crossed continents for tepid snorkelling. But: Mustique. If there is, on the planet, a more meretricious holiday destination, one with the same special flavour of historic slavery, touristic colonialism and untrammelled British snobbery, it remains undiscovered.

Mustique, as the captivating new memoir by Anne Glenconner, once lady-in-waiting to Princess Margaret, coincidentally reminds us, is where her mistress discovered “a whole new life” on land given by Lord Glenconner, then Colin Tennant, a man “utterly in awe of the Royal Family”. Tennant, who was prone to private sadism and public tantrums (including biting), bought the struggling plantation island in 1958, then reinvented it as a swank housing estate.

Shortly, the isle would be full of massive villas, noises and raucous parties, which gave delight and, at least to classy residents such as Princess Margaret, and her young boyfriend Roddy Llewellyn, hurt not. One of her beach parties, Glenconner recalls, featured that most royal of contrivances, “a roll-top bath full of champagne”.

At Tennant’s 60th, an event two years in the planning, “Colin had thought of every detail, even the clothes: we had gone to India twice to choose the outfits. There were T-shirts made to mark the occasion and Indian clothes for the parties, which were laid out for guests…” And maybe posterity will also be more respectful to the Philip Greens.

Mustique’s Caliban and Prospero, the princess and the property developer, thrived in a crudely hedonistic yet otherwise conventional system whereby the “villagers”, as the Tennants called tenants, featured largely as help or, occasionally, accessories. “Colin got some of the local lads oiled up and they wore nothing at all except a gold-painted coconut strategically placed down below,” Glenconner writes. “That night made Mustique famous for ever, mainly down to the golden boys dancing around Princess Margaret. Even for the mid-70s, the scenes were an unusual sight.” The parties, her husband said, “made Mustique famous and therefore more profitable”.

The anonymous islanders, possessing for some reason no “smart clothes”, were costumed in crinolines, striped trousers and top hats for the Queen’s arrival on Britannia in 1977. An earlier Britannia visit had been commemorated by Tennant renaming, in the tradition of all thorough colonisers, the bay now welcoming Johnson. In fact, it doesn’t seem impossible that a new Carrie Inlet or Johnson Promontory won’t join Britannia Bay, after a visit that promises to do as much for Mustique’s profits as Tennant’s grotesque parties. “Still today, it attracts the same sort of people as it did all those years ago,” Glenconner confirms, “from the Royal Family to the social elite.” And as of now, one-nation shaggers.

As little as any reasonable person expected Johnson, once world king, to become something beyond the sum of his appetites and entitlement, there was a chance of performed responsibility, or concealment, such as to increase his party’s long-term chances of re-election (that is, in a future not featuring Rebecca Long Bailey). If you didn’t expect, for a second, to find the career sleazebag hunkered in the Scilly Isles or posing with walking poles, it was possible Johnson might submit, Cameron-style, to occasional short-haul flights to chilly sea, even to economy seats, before holing up in some secret hell containing the sort of people he likes: Lebedevs, Bismarcks, much younger women with blond hair.

Instead, Mustique. An island routinely promoted with tributes such as “the world’s most glamorous island”. As we learn from Lady Glenconner, this hardly does justice to its Tennant-fantasy heritage. There are other resorts fetishising wealth and privacy. “The Queen’s visits cemented,” Glenconner writes, “the idea that Mustique was also a playground for the aristocracy, which was probably, on balance, nearer to the truth, simply because most of the party guests were members of my or Colin’s family, far outweighing the rock stars.”

Hence, perhaps, Mustique’s irresistible allure for Johnson and Symonds: the new, one-nation-conservationist iteration of Margaret and Roddy.

• Catherine Bennett is an Observer columnist