"This is for a 300th anniversary," said Dame Margot Fonteyn, cutting into a large white cake with yellow roses. "I just want to make sure you all know it's not my anniversary."

The "prima ballerina absoluta," as British Ambassador Sir Oliver Wright described her last night, got her start 250 years later than the theater where she first appeared. But like Sadler's Wells in London, the name Margot Fonteyn is synonymous with ballet. So who else but Fonteyn would you expect to be leading the celebration of Sadler's Wells' tercentenial year at the British Embassy?

And who better than Fonteyn, wearing a diaphanous dress that left no doubt her legs are as shapely as ever? She was there presiding at the debut of Sadler's Wells Tercentenary Appeal, as it's called in England, or Sadler's Wells Association Inc., in the United States. And like everybody else in the arts these days, whatever their nationality, the tin cup is out--or almost out.

The party was a toe-shoe-in-the-water to see what support, if any, Americans will offer toward renovation of Sadler's Wells Theatre's stage, estimated to cost upwards of $5 million, and the establishment of a $3.5 million endowment.

"English institutions are all doing the same thing," said Daniel J. Terra, U.S. ambassador-at-large for cultural affairs. "They start American foundations. They get a 501-C3--it's a rating you get that allows everybody, corporations and individuals, to make contributions and get tax deductions.

"They all do it, every one of them," continued Terra, who was Ronald Reagan's campaign finance chairman in 1980. "In Britain you cannot take tax deductions for contributions. This is the only country in the world where you can do that."

But the appeal will not be limited to British companies with American branches, if Sir John Mayhew-Sanders, chairman of the British-based Appeal, and others on the committee find there's sufficient interest.

Part of the pitch will be to remind potential contributors of Sadler's Wells' practice of presenting American dance companies to British audiences.

Mayhew-Sanders, "not presuming to presume" that Americans would want to support the cause, was nonetheless optimistic. (His Scottish-based company, John Brown, has a $182 million contract for the Soviet pipeline project.) He hopes the reelection of British Prime Minister Margaret Thatcher "means like-minded, basically Anglo Saxon people at the moment are thinking the same sort of way.

"It brings along some hardships for people," he said, "but in the long run it's what's going to do the people good."

Whatever Thatcher's policies do for people, Sadler's Wells' Royal Ballet director Peter Wright said he didn't think her policies are going to do the arts any good. "I don't think we should have to rely on sponsorship from the outside to that extent."