The Marriott is a very nice hotel, in case you haven't been there before. I went to a birthday party in the hotel last year, and it's also a good location not too far from Westminster.

Don't look for cheap restaurants. There are none in London worth eating in, even with the dollar being so bad. If you get to the Borough Market (a real experience) on Friday, hit it right before noon and have some good tapas at Brindisi's. The food is wonderful... also at the Lebanese Ishbilia near Hyde Park Corner. Maybe try a dinner of fish and chips at one of the better spots.. Masterfish (not far from your hotel) or the Golden Hind near Oxford St. Haozhan in Chinatown is also very good for lunch.. right near Leicester Square.

Enjoy your trip and I hope you have wonderful weather!