So I've been playing the WoW F2P trial thing. It lets me play as much as I want up to level 20 and with the races from Burning Crusade etc. If I now want to start subscribing to play properly, what do I need to buy? Do I just upgrade to a full account and that's that? Or do I need to buy the game and the three expansions at retail? The Battle Chest thing?

Looking for definite answers, not 'I think you'd do this', please!

You will need a World of Warcraft CD key to be able to level up to 60. After that, a Burning Crusade key to level up to 70. Lich King will let you go to 80, and a Cataclysm key will let you reach 85.

You can purchase CD keys digitally from Battlenet, but I find them to be almost twice the price compared to getting an actual boxed copy from Amazon.

Burning Crusade is now free with buying WoW right out of the gate. After that, maybe wait and buy the other two expansions, (Wrath of the Lich King and Cataclysm) as you reach them to make sure you still like the game.

This is the breakdown of what you get for the box. 
 
Original WoW = Unlock the full game and Burning Crusade(max lvl 70) 
Wrath of the Lich King = Death Knight Class, new zones for leveling from 70-80 
Cataclysm = 5 more levels(currant lvl cap) 
 
You do not need to buy Burning Crusade, it is free with the original WoW. So unless you really want to play a Death Knight, just buy a CD key and worry about Wrath and Cataclysm when you get to those level caps.

go to battle.net upgrade your account, and you can get to lvl 70. Takes like 2 minutes, I think it costs 10 bucks though, or maybe it was more.

Thanks for the help, chaps! <3