I loved the high tea at here:
http://www.afternoontea.co.uk/.....8;Itemid=3
Also, I definitely agree with the suggestion for Indian food, you can't go wrong with the food there.
For quick food. I love the sandwiches from upper crust and the paella at covenant gardens.
If you drink coffee, I'm jealous because the coffee over there (if you like milk) tastes way better.
Also, eat a British breakfast, I love the bangers.
If you do end up doing a side trip to Bath - eat at the Sally Lunn Bunn House
I love London! It's horrendously expensive though!