Fiction: Strangers in a strange land; The London Embassy, by Paul Theroux. Boston: Houghton Mifflin Company. 248 pp.$ 13.95.

There is a small but impressive group of writers who have made it their business to describe the colonial experience: the stranger, in a strange land, representing a powerful and alien tradition, who observes with impartial and ironic detachment the goings-on both of the locals and of his own compatriots. As reluctant inheritors of some imperial responsibilities, American writers are joining this once predominantly British group. Paul Theroux, if not yet quite a Waugh or a Maugham, has the requisite scepticism and wry humor that make his account of Americans abroad tolerant and perceptive. His latest novel, ''The London Embassy,'' is a sequel to ''The Consul's File.''

The narrator, still unmarried, has been promoted from his position in remote Ayer Hitam, to be political officer at the American Embassy in London. Writing of a city and people thought to be as familiar as near kin, Theroux has made London and the English seem as foreign, as inexplicable, and even as menacing as far more exotic peoples.

The embassy is motivated by a spirit of ''us'' against ''them.'' The number two man, Horton, is a former football player who treats his subordinates as a team that must win - goodwill, trade advantages, special knowledge, and useful contacts.

Embassy parties have their prearranged game plan, complete with post-game quarterbacking in which coach Horton evaluates what has been gained or lost. The Ambassador, Noyes (''no-yes'' to the staff), is a political appointee who gives and underwrites the dinners for the important visitors. There is a splendid account of Prime Minister Thatcher and her husband, Dennis, attending such an occasion, which surely owes more to fact than fiction. A guest admits to doing research '' 'on Mary Shelley,' she said, with a hesitant bow to the Prime Minister. 'Frankenstein!' the Prime Minister shouted, and moved on. She had already put herself in charge of us.''

The embassy staff is a predictable collection of characters, for one of the cardinal rules of this genre is for foreigners to have as many foibles as the natives. Dwight Yorty in the Regional Projects office hits his wife on the head with a cucumber, which he then proceeds to eat as she stalks out of the house. Victor Scaduto, the cultural officer and father of three obnoxious Anglicized snobs, schemes to be posted to Italy. Another member of the staff, Miss Duboys, is suspected of spying or selling groceries on the side, for her food bills at the PX are far too big for one skinny woman living alone.

And the natives are no less interesting: A suburban spinster is troubled by her fanatical but thieving Muslim boarder, who, she is convinced, must be an American; a young woman of impeccable breeding turns out to be a ruthless money grubber; and an ancient but impoverished baronet is pursued by the embassy, because titles are considered important.

Each chapter could stand on its own as a self-contained short story. But the changing seasons, a meeting with a beautiful and accomplished heroine, and the narrator's marriage to her at the end give the book the coherence of a novel, though it is a marriage that seems arranged more for the author's convenience than the characters' real needs.

Theroux has a keen eye for those absurdities that seem more striking when different peoples are brought together. Life abroad is as teeming with variety as any drop of pond water viewed under a microscope, and Theroux makes the most of such an opportunity. But at times it seems as if the phenomena have been observed on too many other occasions, the tone is a tad stale, and Theroux's response, like that of his foreign-officer narrator, is one of duty rather than inspiration.