President Reagan stood on one foot, then the other waiting for Nancy Reagan and the wife of the British ambassador to come up for air the other night at the White House.

Some of the Reagans' guests at their diplomatic dinner were not so patient, however, and asked State Department aides if it was all right to leave before the Reagans did.

It's safe to conclude now that the first lady and Mary Henderson were talking in part about Queen Elizabeth's invitation. The Reagans will stop in London during their trip to Europe in June. And Lady Henderson was helpful the last time her good friend Nancy went to London. That, of course, was when Prince Charles married Lady Di.

One report is that the queen has invited the Reagans to be her house guests at Windsor Castle, which would be a coup by anybody's standards. But yesterday, before she flew to Florida, Nancy Reagan said she didn't know yet what the arrangements will be in England.

The East Wing staff is developing a separate schedule for the first lady when she is abroad and one visit to Italy seems almost certain. That's a stop at a drug rehabilitation program patterned after Daytop Village in New York, which Mrs. Reagan has visited. Involved in the program is Ruth Rabb, wife of U.S. ambassador to Italy Maxwell Rabb.

The wife of the ambassador of Luxembourg didn't let a little thing like a broken arm cramp her style the other night at the diplomatic dinner the Reagans gave. Candace Meisch wore her cast to the party, wrapping it up in black lace and tucking a pen into her evening bag. ("Are you kidding?" she said later. "Do you think I was going to miss a chance like that my first time at the White House?")

Finding herself on the president's left at dinner, she asked for his autograph. He obliged with: "To Candace with best wishes, Ronald Reagan 2/ll/82." Two more autographs (George and Barbara Bush's) later, she approached Nancy Reagan in the Blue Room who wrote: "To Candace with best wishes, Nancy Reagan." "Don't erase mine," kibitzed the president.

Taking it all in with typical wry amusement was the Luxembourg ambassador, who started it when he gave his wife a 12-speed bicycle as a combination Christmas and birthday present. Three days before the dinner, out riding with Yves and Patrick Meisch, Candace fell off and broke her right arm.

"An historic cast," mused Adrien Meisch, who decided that if she had to break her arm she did it at the right moment. On the other hand, he added, "I gave her the bicycle but not for that."

On the president's right at that same dinner was the wife of the Egyptian ambassador, the second time in a week Amal Ghorbal had been seated at his table. The first time was at the official dinner the Reagans gave for Egyptian President Hosni Mubarak. "I just changed seats this time," said Amal, who probably set some kind of historical record. Her husband Ashraf, though No. 7 in the order of diplomatic precedence, turned out to be the evening's ranking ambassador.