Whilst.
Listen asshole, you aren't British and you never will be. Who are you trying to kid with that "whilst" business? What, suddenly you're continental and sophisticated because you added a hiss and a flick of the tongue to the end of an ordinary word? Bra-fucking-vo. You probably own a tweed blazer.
Did you grow up watching the BBC and pretending that Marmite doesn't taste like meningitis? Do you call aboriginals "niggers" and have a snaggle tooth? Is your skin translucent? Do you insist that their exist micro-regional variations in British accents, down to the street? Does your telephone have a shitty two-pulse ring? Do you claim that your ancestors "invented" a language that your citizenry is vehemently destroying on a daily basis? No?
In that case, when you see the words "chiefly British" in the dictionary, respect them — you fuckin' twat.