I can fake an English accent enough to fool Americans and fellow Canadians, but it’s still not up to snuff for fooling actual Englishmen. I have a bit of an unfair advantage though, my grandmother was from Kent, so I know what an proper accent should sound like. Still, I’m trying to fake it over an Albertan accent, which is a normal Canadian accent with an unfortunate hint of American twang.
Two tips; first, make sure to have a working knowledge of British pronunciation and slang, it’s “zed” and “al-u-min-i-um”, not “zee” and “alu-mi-num”. Secondly, watch plenty of Britcoms or the BBC, you’ll get the hang of it eventually.