It's more down to what you do than if it's me or them. Doing stuff the locals do is not tourist. Queueing to take photos of the changing of the guard or that kind of stuff is touristy.
Which part do you oppose the most? The queuing, the photo taking, or the finding of the changing of the guards an amusing spectacle worth seeing once in your life. If there's no queue and I don't take any pictures, is it still "touristy"?
The bottom line is that most "touristy" stuff became touristy because it was a kind of cool/unique/fun/interesting thing to see/do. I agree that if there's a long queue you have to balance how much time you want to spend against the value of the thing you'll see.
The one where you lift property prices so high that nobody can live in the city centres anymore.
Dictionary definition of a tourist: "One who travels for pleasure.".
This local goes the same places the tourists do because the UK is beautiful and the foreign tourists I meet have good taste ;)