What are common tourist mistakes that visitors to the UK make?

I live in Italy, and an extremely common mistake that tourists make is to eat at crappy tourist trap restaurants. They just figure "well shit, it's Italy ain't it? We'll just hit any ol' place." Unfortunately, that's not how Italy works.

The locals have their own hidden away places (it's often the case that the more tucked away and unassuming it is, the better it is), so they are personally unaffected by the existence of the tourist traps. Unless the tourist does a fair amount of homework before getting on the plane, the odds of them discovering the good places on their own might not be so high. So they get suckered into eating at the crappy places, they feel cheated and get really pissed off, and then they go home and complain all over Reddit.

What about when they go to UK?