Sir Nicholas Henderson, who becomes London's man in Washington this July, is aptly described by a friend here as "a type-cast British ambassador." He has combined an orthodox meritocrat's progress as Britain's most successful career diplomat with an eccentricity of both professional and personal style that Americans associate with English aristocrats.

Somewhat shambling and donnish in appearance, his dark diplomat's suit not quite fitted to his tall, lanky frame, tie slightly askew and thinning gray hair pushed carelessly to one side, Henderson nevertheless is known for his stylish elegance as ambassodor to Warsaw, Bonn and Paris.

He is diffident in manner at first meeting, worrying aloud that he might be boring an interviewer who is clearly too inexpert to be sufficiently challenging. Henderson is a diplomat of unusually strong opinions and is never slow to express them no matter what the consequences.

Henderson found it frivolous to spend time talking about diplomatic party-giving, and would say only, "We've never seen why entertainment should not be entertaining." But he and his lively Greek-born wife, Mary, are warmly remembered in European capitals for the good food, great fun and brilliant conversation of their embassy parties.

Unashamedly patriotic about Britain, where he now sees "evidence of a new mood and spirit," he boldly staged British fashion shows in the embassy in Paris and promoted British garden tools at lawn parties.

passionately pro-European, he undiplomatically lectured the Labor government of former prime minister James Callaghan from Paris, about its sour relations with the rest of Europe, but Henderson sees no conflict between his being "an undiluted European" and his new role as an important link in the traditional "special relationship" between Britain and the United States.

The new Conservative government of Prime Minister Margaret Thatcher, who asked Henderson to come out of retirement to go to Washington, "attaches great importance to Britain's role in Europe," Henderson said. "But I see no incompatibility between that and good relations with Washington."

A stronger Europe, in Henderson's opinion, can lead to a stronger Britain, which in turn would become a stronger partner of the United States. He believes both Britain and the United States can learn from the recent experience of West Germany and France, which he saw first hand the past seven years, first in Bonn, then in Paris.

"A lesson can be seen," he said, "in how swiftly France and Germany turned themselves around. It shows what a change in will can do. Leadership is not a very fashionable word, but I believe that leadership in attitude and approach can be seen there. They are still extensively democratic, but there is a sense of national purpose."

Henderson believes he has seen the first early evidence of a similar "new mood and spirit" in Britain, expressed in last month's vote for the Conservative Party and its promise to encourage individual initiative and hard work. A change here, he said, could lead to Britain's projecting "an invigorating new spirit abroad."

Because he believes the United States, like Britain, "is really one of the oldest industrial countries in the world," and that both must develop high-technology alternatives to the heavy industry both are losing, Henderson said one of his particular interests as ambassador to the United States is encouragement of "some form of high technology partnership between the two nations, he believes Britain still has "a lot of skill and inventiveness" to be exploited.

Henderson has a reputation for remaining aloof from his embassy staffs and relying more on his own observations and judgments for his notably personal, pointed and frank reports back to London. He insists he has always consulted with embassy experts, but, he adds, "One's own judgment is what one is paid for."

"I believe that one should report everything exactly how one sees it, no matter how unpopular it might be," Henderson said. "I grew up in the 1930s when the tendency of both British and U.S. diplomats was to fudge what was going on in Nazi Germany and the countries around it, because the truth was thought to be unpopular at home. There are still lessons in those years to be studied by any post-war diplomat."

Although Henderson's individualism is known to be unpopular with some in the Foreign Office who cannot understand how such a nonconformist could be so successful, his rise has been misleadingly conventional.

"He is that familiar paradox of British life," journalist Patrick Keatley observed in the Guardian newspaper here. "The Mandarin whose background is so impeccably orthodox that he can afford to be highly unorthodox when he chooses, and damn the consequences."

Born John Nicholas Henderson, the son of a professor, he was educated at Stowe School and Oxford University. He is still known to colleagues and acquintances in Britain and Europe by his schoolboy name, "Nicko."

From a start as a private secretary - a glorified staff aide - to Britains's War 11, he worked in British embassies in Washington (from 1947 to 1949), Athens, Vienna, Santiago and Madrid before becoming ambassador to Poland in 1969. His subsequent moves to Bonn and Paris could well have led ultimately to Washington, except that Henderson reached 60, the mandatory retirement age, in March.

He and his wife came home from Paris and began redecorating their London home in fashionable Kensington and their Berkshire country cottage, which actually is a string of cottages they are having remodelled. But when former Conservative prime minister Edward Heath turned down Thatcher's offer of the Washington post, the most prestigious in the British Foreign Service, she immediately asked Henderson to come out of retirement.

"We were going to stay in England," Mary Henderson said. "But we are dilighted to leave for Washington. I simply love America and am looking forward to going there tremendously."

She worked in the United States as a correspondent for Time Magazine, for which she helped cover the Greek civil war. She said she has since become more interested in cooking, and experimented in Paris with combinations of her British dishes and those of the embassy's French chef. A book based on the results - "Mary Henderson's Paris Embassy Cookbook" - will be in American bookstores not long after the Hendersons move to Washington. CAPTION: Picture 1, Sir Nicholas Henderson, by UPI; Picture 2, Sir Nicholas Henderson, by AP