GM crops are already in the UK. And there's nothing wrong with GMO. The "organic" movement is a hustle and an industry itself.
Agreed, though, that a lot of things from the U.S. are not good for the UK and should not be imported. The health care "system" is first and foremost. Chlorinated chicken will be another but that's the way it will likely go outside of the EU.
I'm surprised that the UK readily adopts so many cultural traditions - like the OTT Halloween, or prom. But it's not like the U.S. is forcing those on anyone. Living over there now I've grown to quite like Halloween, or at least not mind it, thanks to my wife. A parade of children in imaginative costumes is hilarious. We always set up a bunch of stuff for them. Talking pumpkins and what not. Nobody acts like an a-hole either, which is more than can be said for some of experiences back home.