You'd be suprised mate, with Steam we get gouged pretty bad here, games are never charged based on exchange rate. They charge almost the standard RRP (which for PC games here is $89.95) on a lot of things on steam. So a lot of games that are $49.95 in the US store will be $69.95 or even more in our store, some are even charged at the full RRP of $89.95 meaning there's practically no advantage to buying it digitally. This is regardless of the fact that our dollar is currently a few cents stronger than the USD.
I hope you guys aren't going this route, I'll be buying it anyway but please don't make me import a copy from the UK, I really want to play it day 1