London
If you're looking for a bite to eat that won't take too much time or money, consider PRET A MANGER; here's their philosophy:
very good for "fast" food - they also have a location at Heathrow
"You won’t find ‘sell by’ dates on our fresh sandwiches and salads. We don’t sell ‘factory’ stuff. We offer our food to charity at the end of each day rather than keep it over.
We shun the obscure chemicals, additives and preservatives common to so much ‘prepared’ and ‘fast’ food. Our ingredients are delivered fresh every day. We don’t mind that fresh and fragile ingredients go off quickly – we start from scratch each morning. Our menu is updated all the time, so check back often."
When I think London, I think Buckingham Palace with its Queen Victoria statue out front, the changing of the guards (although I like the changing of the horse guards at Whitehall and the sounds of horses coming down the cobblestone streets).
We enjoyed seeing the Rosetta Stone and the Greek Temples at the British Museum - the Brits did a great job of bringing home some great souvenirs

while ruling the world.
And if you need to use the Loo, the McDs in England are pretty far out - many of them decorated 1970s style.