Of course we are welcome to go to the UK. It's not like we are bringing them more COVID. They already have plenty of it to go around. The question is all about whether you think you'll avoid getting infected while you are there.
If you want to go to New Zealand, it's a different ballgame: for them, Americans are the bigger risk, so quarantine it is.
I am supposed to go back in September / October, yet, from everything I am reading, that will be more risky than it was in June because (a) the Delta variant has spread further and (b) the efficacy of the vaccines seems to be wearing down over the course of a few months, especially against Delta. The data on that is still evolving, but thank goodness other countries like Israel are tracking it, since the USA decided not to.