The trouble is that it is not a simple currency conversion for the UK.
MS aren't offering the upgrade versions at all, but are offering promo prices on the retail version.
For instance, Home Premium £50 = approx $83 at current rates. There are advantages for some people with this. Still have to see what sale prices will actually be. Compared to prices in the past, the pre-order pricing is very good and MS seem to have the marketing right on this to get W7 off to a good start, unlike Vista. Feel sorry for anyone who paid good money for Vista, but that's probably what has got the rest of us better prices now.
The long-term prices, like-for-like, are of course higher in the UK, but the % seems to vary a lot according to the version.
Not sure how easy it will be to access US versions, or how well they will install/activate. Will be watching out for any info on this.