"We can change the sheets, but what we can't do is cool the mattresses."

Such is British Ambassador Sir Oliver Wright's assessment of the bed-and-breakfast business at the British Embassy, where the parade of officialdom seems endless these days.

This week, for instance, the British are sending not just one but two Maggies. Prime Minister Margaret Thatcher arrives tomorrow for an official working visit with President Reagan. Princess Margaret comes on the weekend to join the Reagans for dinner.

Not since another royal, Prince Charles, visited in 1981 has such an intimate White House party been planned by the Reagans. At least three eligible bachelors are on Saturday night's guest list: Sen. John Warner (R-Va.), ballet star Mikhail Baryshnikov and Sen. Malcolm Wallop (R-Wyo.), whose British connection includes an uncle (the Ninth Earl of Portsmouth, Gerard Vernon Wallop) and a brother-in-law and sister, Lord and Lady Porchester (Jean Margaret Wallop).

The night's entertainment will be courtesy of someone else with a British connection--Marvin Hamlisch. He's due back this week from London, where his newest musical, the story of Jean Seberg, is in the works. On Thursday night, Hamlisch will be celebrating the 3,389th performance of "A Chorus Line" on Broadway.

Others among the 32 guests are likely to be former ambassador to Great Britain Walter Annenberg and his wife, Lee; Evangeline Bruce, whose late husband, David, was also ambassador to Britain; National Gallery Director J. Carter Brown and his wife, Pam; actress Claudette Colbert; Drue Heinz, who entertained Nancy Reagan the week of the royal wedding; cosmetics priestess Estee Lauder, and White House Deputy Chief of Staff Michael Deaver and his wife, Carolyn.

Margaret comes to town via the Caribbean, where she attended the independence ceremonies of St. Kitts-Nevis.

From Washington, she reportedly goes to an undisclosed Virginia destination to stay with friends. Both Wallop and Warner have homes in Virginia, but their offices said neither had tendered an invitation.