Well you'd be surprised actually...
A lot of American actors can do very convincing accents (to my ear) and I have lived all over the UK, each place for several years. Most of the american actors go for either a posh london accent, or a more cockney london accent. But the UK is full of dialects and accents. Somerset, Bristol, Birmingham, Nottingham, Sheffield, Manchester, Liverpool, North and South Wales, Glasgow, Edinburgh, Newcastle, Leeds, etc They all have distinct accents that are quite different. Would like to see them try a geordie accent!
For my money, the nominations for "most convincing brit accents by americans", in reverse order, go to:
3. Gwyneth Paltrow in Sliding Doors.
2. Johnny Depp as Jack Sparrow.
1. Without contest....All the american comedians who did Spinal Tap
. They utterly fooled me at the time, I had no idea they were American when I first watched it (As we didnt even know about SNL) So authentic, they must have been so well coached.
The worst goes to:
3. Richard Gere for murdering the Irish accent in The Jackal
2. Ewan McGregor's american accent in The Island (it works both ways!)
1. Dick Van Dyke's abysmal "cock-nay" accent in Mary Poppins