Dont think your missing the point. If you cant use it in the UK then Amazon US and all the other US sites wouldnt ship it here. Software is more expensive over here due to VAT, The exchange rate, and greed and the UK retailers know we are willing to pay the higher prices, and its not just retailers, M$ sell it to the retailers at a higher price too. They use some sort of price index to determine how much it will be sold for.
You can buy it from other countries too, i would have thought some eastern countires will be cheeper that the USA.
If MS didnt want it sold on to other countries, they would have build in something to stop it been purchased from different countries, lets face it, VISTA is full of security and anti piracy features.
I got vista for free from MSDN anyways, if your company has an MSDN subscription you can download a copy with a key from MSDN and save £150 or so, its the Pro version only on there though. Or if you are at Uni, AAMSDN does it for free too.