For my husband and me, food is generally fuel to keep us going when we're traveling. We were in London last year and dropped into Sainsbury's a lot- it's a grocery chain that has selections of sandwiches and other food to go. Not very elegant, but it doesn't break the budget and it gives you a few pounds to spend someplace a little extravagant. In our case, it was a dinner at Simpson's-in-Strand!