Many of the suggestions above are v. apt, in particular from ajax. However, if your wife is a foodie, and you are willing to have at least one meal that isn't budget, there are absolutely wonderful restaurants in London that do extremely good value (for London) fixed price lunches. You will still be looking at c. GBP50/head, but given that these are world-class restaurants, and that a mediocre meal in many places will set you back GBP30/head, this is fantastic value imo.

tb