Discount Britain (and similar offers) are fine if you're sure that you really want to spend most of your time visiting museums (and other tourist attractions) which charge an admission fee. You also need to be aware that some of the places they give discounted admission to are often well outside the central London tourist area. The only way to know if these discounts will be worthwhile for
you is to plan a schedule of visits and see what you'd save.
Normally, to save money on travel in London, you should buy an Oystercard. However, if you intend to take some river trips, you should be aware that many river operators give you a third off their fares if you present either a Travelcard or an Oystercard charged with a Travelcard. (An Oystercard charged with cash doesn't get you the discount). So it might be worth buying a Travelcard for one or two day's travel.
For a really good 3-course meal, at really cheap prices, try a branch of Bistro 1:
http://www.bistro1.co.uk/
(For example, at lunchtime, you could have a starter of Smoked Duck salad served with mixed berry coulis, a main course of Fillet of Sea Bream served with mashed potato& smoked salmon with saffron cream sauce, finished off with Tiramisu for just �7.50).
Chris