I haven't been to Paris or Madrid, but I like Barcelona better than London. Both are great cities and easily will entertain you for a week, but Barcelona is just a bit more different than America than London is. Great Indian food in London, and British food gets a worse rap than it deserves, but it doesn't top the food in Barcelona which is one of the best eating places I've been.
If you do Barcelona, take a bus or rent a car and go up the coast to Tossa de Mar for a couple days.
London is unique, you either hate it or you enjoy it.
Im okay with it, lots of stuff to do, if I lived in the city, pretty sure, it would grow old.
I havent been since 91, so, Im sure its changed alot.
There was a Chicago house of Beef, located down an alley and in the basement of some people. I probably wouldnt have found on my own, that was by far, the best meal in England, I had in 3+ years.