It's basically the same for a US citizen moving to the UK as for just about anyone outside the US moving to the US. Look at all the anti-immigration sentiment in the US and the reasons given for it. While it may be anti-Mexican in the US and anti-Magrebhi in France and anti-Asian in the UK, it all boils down to the same arguments: "They" aren't like us. "They" will take "our" jobs. "They" only want to come over here for the welfare benefits or health care system or schools. "They" don't pay their fair share of taxes. "They" live off our economy and then send their money home. "They" don't speak our language and expect us to learn theirs. etc. etc.

Go visit England for a few weeks or a couple of months and then decide whether or not you want to move there for the longer term. It's certainly not impossible, though it helps to have a transferable profession that is in demand in England (or whatever country you're looking to move to). But all countries these days do want to keep the riff raff out.
Cheers,
Bev