IN 1912 WHEN HE was 38 years old and going nowhere as a published writer, Robert Frost chucked his job teaching school, sold his New Hampshire farm, gathered up his wife, four children and his Morris chair and moved to England. They went, he later wrote, for "the cheapness of peasant live under a thatch." And for a more practical reason - to be launched as an American poet. Although Frost arrived in London without any conncections and no letter of introduction, he found, almost at once, a publisher for his poems and a promoter for his career - Ezra Pound, self-appointed impresario of the expatriate literary colony. Three years later, when Frost returned home, he was as well-known as any poet in America. England, he later said, was "the country that has made me a poet."

American writers and painters who found in London a professional springboard - and a refuge - in the two decades preceding World War I are the subject of Stanley Weitraub's book, The London Yankess. His is a panoramic view of 20 vivid years. His wide-angle lens catches, at the far end of that period, the combative James Whister in the twilight of his career (complainting that he "hardly had a close enemy left in London") and, at the near end, the last great incoming wave of young American expatriate writers - Ezra Pound, Robert Frost, T. S. Eliot - all of whom broke "the barrier of American indifference" to their work by first publishing in England.

Also on the horizon are "The Master," Henry James, past the prime of his career but still the most commanding figure on the literary scene, and John Singer Sargent, already weary of the burden of being the foremost portrait painter of his time. "Ask me to paing your gates, your fences, your barns, which I should gladly do, but NOT THE HUMAN FACE" he wrote a would-be sitter. Mark Twain turns up, stopping off in London for three years as part of his nine-year exile from his country and his creditors. Now in his mid-sixties, he is more visiting celebrity and popular after-dinner speaker than working author. Also on hand are Stephen Crane and Bret Harte, both already famous, both entangled in complicated domestic arrangements and both writing to specification - strictly for money. (The editor of the Graphic once asked Harte to write a story, any story, as long as it contained a real live bear; he obliged.)

Weintraub's assembled case is astonishingly large, colorful and feisty. Despite differences in age, style and degree of success, these Americans mixed in an atmosphere of easy camarderie - helping each other, advising, editing, cajoling and sometimes quarreling. Unlikely firendships were struck. Henry James took an interest in the much younger Stephen Crane, bicycling from Lamb House to Brede House to call on him. They were as unalike in style as two writers could be: Stephen Crane carried on like a Bowery boy, killing flies with his Colt revolver; Henry James was elegant: and a shameless snob. (Clarence King once wrote that before Henry James ventured into obscure part of London, he "gathers up a few unmistabeably good invitations and buttons them in his inner pocket, so that there should be no mistaking the social position of his corpse if violence befell him.")

Most arresting of the vignettes in this compendium is that of the difficult, irascible but oddly appealing young Ezra Pound who arrived in London at 23 in 1908 with three pounds in his pocket and a conviction that the "only way I could educate the educable minority in the United States was to come to London." He bot himself published and then took on the job of boosting other young American writers and artists - among them Frost, Eliot, William Carlos Williams and Jacob Epstein, the sculptor. "He seemed to be sponsoring, or editing or contributing to, or translating, or adapting, or advising, in every corner of literary Lond." Sometimes he overdid it. As Frost later said: "Pound took a poem of mine and said, "You've done it in fifty words. I've shortened it to forty-eight." I answered, "And spoiled my metre, my idiom and idea.""

Mixed among these all-stars are many lesser players - Henry Harland who edited the Yellow Book, Pearl Craigie who wrote novels (and was the Carter's Little Liver Pills heiress), Harold Frederic, the New York Times correspondent who was best known for having two sets of "wives" and children in England and becoming the central - and posthumous - figure in a manslaughter trial - recorded here in more detail than it may deserve.

Wintraub's eye moves so quickly over such a broad scene that occasionally the impression of 20 years of American talent rolling by seems blurred and confusing. But if this episodic slice of literary life at times lacks depth, it still offers much that is interesting about that long-ago time when to become an established writer or painter it was first necessary for an American to be an expatriate in London. CAPTION: Illustration, no caption, Jacket art from "London Yankees"