Thiefsie at rps.com found a nice way to get the steam games in pounds.
I tested it and it works!
1) Put ?cc=uk after a title. You'll now see the prices in pound.
2) Change your country to United Kingdom.
That's it, you can now buy the game in pounds.
Possible to save quite some money this way. Left 4 dead 2 costs €37.49 compared to the us £22.5(~€24.5). And the THQ complete pack costs €49.99 as compared to £26.49 (~€29.2).
As always with these things: use at your own risk. It's probably not going to last very long.