More exposure to the accent. We brits grow up watching US TV shows from birth. Americans watch Mary Poppins and assumes that’s how we all talk.

I think Renee Zellweger does a very passable English accent as Bridget Jones

I hear horrible attempts at US accents all the time. The fake accents on some episodes of Doctor Who have made me laugh out loud.

British ppl don't do an American accent, they do NO accent. Dr house was not a GOOD American accent, for example

It’s likely that if you’re British, you are hearing flaws in the American actor’s British accent that the American can’t hear. And you think the British actors have great American accents because you are unable to hear the flaws an American would hear.

You’ve obviously never heard a Britt attempt a southern accent.

in the UK there are hundreds of different accents all within small distances of each other because we are such an old country. USA is a relatively new country with a new english accent, there's less diversity and more simple to recreate

I'm not sure that's true

This isn't actually the case. I've only heard two Brits whose American accents were actually convincing, and that's Christian Bale and Gary Oldman. Hugh Laurie is an honorable mention.

You are 100% correct. American actors are usually dreadful at doing regional American accents as well; The Brits typically do a southern US accent 10 times better than an American actor .. British actors research it, and put effort into it. American actors assume they know southern accents - they seldom are good at it.