I had optimistically stepped off the plane in London without a hotel reservation and four hours later I was still rejecting rooms and revising my vacation budget.

Singles at the local Holiday Inn were going for a staggering $290 a night and the half-dozen "budget" lodgings (under $110) I had looked at were dingy cubbyholes with stained carpets and peeling wallpaper.I checked at hotel where a sign behind the desk listed the single-room rate at 59 British pounds - the equivalent of about $120.

"What could you do for me if I stayed a week?" I asked the desk clerk - confidently, I hoped.

She riffled through some papers and replied, "Well, I have a room I could give you for 40."

The room was tiny but cheerful and newly decorated.

The rate, which figured out to about $80 a night, also included a generous breakfast daily. Not a steal by any stretch, but with London's prices these days, I felt lucky.

If ever there was a time to opt for a package trip including hotels it is now, with the dollar worth only about half the British pound. Airline tour desks and travel agents should be able to come up with numerous packages, many including theater tickets and sight-seeing, all with guaranteed dollar rates.

One current deal from European Car Vacations even offers a lower rate than I paid at the same hotel, the Eden Plaza - with a private bath. ECV's per person double rate for a three-night package there, including round trip airfare from New York, is $609. Extra nights are $53 (the single rate is $669; extra nights, $71).

Packages at other hotels run from $599 to $649. The rates are good to March 30, 1991, and include breakfast, transfers between the airport and central London, tips, taxes and discounts at hundreds of shops, restaurants and tourist attractions. Other packages are available, as well, some with rental cars. Information: (800) 223-6764.