We've just come back from London. Had a great time! I promised myself that I would post this as soon as I got home because of all the stress I had about what to pack for my vacation.
I definitely didn't pack white trainers (even tho they are my most comfortable, worn-in and supportive shoe). I bought myself 2 expensive pairs of black walking shoes. I wore them to work for weeks to break them in before our trip. Guess what I saw all over the street of London? White trainers! Of course, they had to be tourists! For the first couple of days, I'd sidle up to these people in order to hear them talking to each other... many were British!! My expensive walking shoes gave me blisters that I'm still recovering from. I really wish I had brought my white trainers. I would have fit right in and been comfortable!
Also it was very hot, especially in Paris where we went after London. Many, many people in shorts.
What I will agree with is that no one wears sweat pants or hoodies.
I was surprised that very few people dressed for the theatre. We saw shorts and jeans there.
My best advice to the 'what to pack' question is to wear what you would wear to walk around a city at home. You WILL fit in! I'm hope I'm saving someone the stess I felt leading up to the trip. There was absolutely no need for it.