The British English word for a bottom is very definitely "arse" - it's an old Anglo-Saxon word (the German equivalent being "Arsch"). In most places around the UK it would be pronounced with a long 'a', to rhyme with "farce". That pronunciation is certainly "The Queen's English" (were that word ever to pass her sainted lips), though in some areas, notably in the north, the "a" will certainly become shorter and flatter (I can't think of a word to rhyme with it though!) But the "a" would never be as short and flat as "Donkey", as in "lass"...
...
except that these days, such is the quantity of US telly that we're exposed to over here, that you'll get youngsters talking about their "Donkey" because that's what they hear on telly. (My son was talking about a 'Car park' the other day, FFS!). And the other thing is that - really more in days gone by, you might get someone being called a 'silly Donkey', which unlike "stupid arse" would not be at all vulgar, because the meaning would be "silly donkey". But the sort of thing you'd only ever hear that from someone rather posh in a 1950s B-movie.
So, in short... it's complicated.
But one thing's for sure - the only place in the known universe that you'll see mention of an "arse plug" is on this very forum, courtesy of its magical USA-filter. I can confirm that the word the Brits use for that particular implement is that same as the US'ians
All clear now?