London — To the thousands of American tourists marching down the leafy Mall that leads to Buckingham Palace, here is some advice: They don't change the guard at Buckingham Palace every day. Only every other day.
''That's what I tell people on my bus,'' says Maurice Page, tour director of a company operating double-decker sightseeing buses. ''If you want to know which day, pick up the telephone and call Buckingham Palace. The Queen won't answer but a spokesman will.''
Mr. Page says Americans are delighted to go home and say how they called the palace. He claims that it's the ceremonial things that Americans like best about London.
For many, the big inducement to visit Britain is the soaring value of the dollar against the pound.
The result: the largest-ever invasion of United States tourists.
Even in cosmopolitan London the Americans are readily identifiable - by their accents, their crisply clean alligator shirts, their sneakers, and their questions.
''They ask, 'Where is Parliament Square?' 'Where's Churchill's wartime bunkers?' 'Where's McDonald's?' - that's the favorite question,'' says a bemused traffic warden standing inside the Big Ben clock tower.
Jim Vornberg of Dallas, who was last here five years ago, says: ''Food definitely seems cheaper. The pound was $2.15 the last time I was here. Now it is about $1.30 or less. It means I can get a good pub lunch for less than $2 or
Len Lickorish of the British Tourist Authority expects at least 2.8 million Americans in '84. More than 13 million tourists are expected in all. This means about $6.5 billion for Britain - putting British tourism in the same league as France and Italy, and ahead of West Germany and Switzerland.
At the top of the top of list of things tourists like here is the British people, the Tourist Authority says. A mini-survey by this newspaper seems to corroborate this.
One peeve of US tourists is that Britain has been too warm - with temperatures in the high 80s. Some Americans say lack of air-conditioning makes hotels uncomfortable. On the other hand, more hotels and restaurants have icemakers and serve ice water - almost unheard of a few years ago.
Mr. Page says the key to satisfying Americans is appreciating they are ''totally service orientated.''
''In the States,'' he explains, ''the waiter can serve you a burger and he'll come back five times to see if you're all right. Here you can sit and they'll forget about you.''