When asked why Britain had decided to salute one of its former colonies, David Lloyd-Jacob, a British businessman who conceived the idea, said: ''During your Bicentennial celebration, some of us British began to think that maybe we had made a mistake in 1776.''
He smiled and added: ''We decided what we wanted to celebrate was the peace treaty of 1783, when the fighting stopped and we finally sailed away.''

The party also served as the opening of ''Constable's England,'' an exhibition of paintings by the British artist John Constable. It is one of the 190 British events ranging from ballet to film to polo that will be held in the five boroughs beginning this week and running into the summer.
The party probably drew more dukes and duchesses, lords and ladies and earls and countesses than have been assembled in one New York location in a long time. The guests included the Duke and Duchess of Wellington, the Duke and Duchess of Marlborough, the Earl and Countess of Westmorland, Mary, Viscountess Rothermere, Lord Montagu of Beaulieu, the Duke of Norfolk, Lady Carolyn Townshend, and Sir Oliver Wright, the British Ambassador to the United States, and Lady Wright.

But as the guests arrived, about 30 demonstrators who oppose British presence in Northern Ireland stood behind police barricades on Fifth Avenue shouting and waving banners.
The Duke of Marlborough, whose ancestral home, Blenheim Palace in Oxfordshire, is one of England's leading tourist attractions, was asked what he thought the festival would accomplish.

''From the English point of view, and especially my own,'' he said, ''I hope it increases tourism and trade from your country.'' He added that Sir Winston Churchill, a descendant of the Marlboroughs, was buried on the palace grounds. ''He was born there, proposed to his wife there and is buried there,'' he said proudly.

The guests included members of the American version of nobility - the rich and the famous - including Brooke Astor, Blanchette Rockefeller, Walter and Betsy Cronkite, John and Mary Lindsay, Marietta Tree, Betsy Bloomingdale, Angier and Robin Biddle Duke, C. Douglas Dillon, Leonore Annenberg, Mildred Hilson, Marife Hernandez, James and Candace van Alen, and Robert and Joan Tisch. Also invited were assorted ambassadors and former ambassadors from both countries, as well as Cary Grant, the actor, who was born Archie Leach in Bristol, England.

Mr. Grant, who escorted his wife, Barbara, was asked whether he felt British or American. ''I feel like a human being,'' he replied. Mr. Grant is a board member of Faberge Inc., one of the party's sponsors.