I've never heard someone who isn't from Britain, do a good British accent. Mainly because all over Britain, people have different accents as it is. You could no way say that a Scottish and a Welsh accent sound alike. Mike Meyers does a feasible Scottish accent though.
Whenever I go to Florida and people say they can do "my cockney accent" and they really can't Lol.
I would love to say I can do a good french accent, but I can't. I can speak french though
I can't do it right, but if you say an amount of money (eg 5 thousand dollars) in your accent and then say it in a british accent, it sounds like more money.