You were (still are) paying less than usual, because you were getting the European 'E edition' version, that was to come without Internet Explorer. As there was no way to upgrade regular copies of Vista to the IE-less E edition, Microsoft decided to sell the full version of Home Premium for upgrade prices. This meant that you could pick up a copy of the OS for £65 (as the others mention, you could pick up a copy for £50 for a brief time).
Since then, Microsoft have decided to ship the OS with a choice of browsers (including IE), which eliminates the need for an E edition, and the need for discounted pricing. On September 1st Microsoft was meant to replace the 'E edition' with the full version (which was going to sell for I think £125) and an upgrade version.
However for the time being, you can still pick up a 'full copy' of Home Premium for the same price as before. In fact at Amazon...
So I would buy a copy ASAP before things change.
If you do miss the window however, note that the upgrade version will let you 'upgrade' from XP, albeit in the form of a clean install (I believe you just have to enter your XP CD key into the installation program).