It's part of the 'Orange Box', which is HL2, HL2Ep1, HL2Ep2, TF2 and Portal. But since the others haven't been released yet the TF2 is being opened up to those who have pre-purchased it as a beta program. I don't know if they plan to have TF2 available separately, but the whole bundle is a bargain at $44.95, so about £22.50 + VAT, and if you already have HL2 or HL2Ep1, you can 'gift' them to another Steam user as a present.
edit: They list the worth of the TF2 part of the Orange Box as $29.95, so about £15 ... and yet you can't purchase it separately, however it may be taken as an indicator that they plan to allow people to purchase that on it's own at some point in the future.
So far I have been incredibly impressed with TF2, the nice thing is when someone develops a tactic there is enough balance to create a counter to it. Everytime you think you have an upper hand the opposition respawn with a few people to mess up your plans. Oh .. and the comedy factor is incredible ... the voice over people are great.
) free, as the web proxy here is pretty strict.
