I think I'm ready to start playing WoW.  
I went to the shop to buy Cataclysm and despite the subscription fees, I was fully prepared to pay the £25 for it but when I looked at the box I see I need to buy all previous versions of it as well. I turned it down in the end not sure how much I'd end up spending. 
 
What is the cheapest way to get all sets and get started? 
 
Thanks for the help. 

Yeah, try out the trial. Takes about 30 minutes to get in game depending on your connection. After that, you can just upgrade your character into a retail version one so you don't actually lose any progress you made. I'd say you should just buy the vanilla WoW (or the battle chest) after the trial if you're going to continue. You'll get the cataclysm-vanilla content "for free" without risking any money by paying for 3 expansions before you're adequate level to explore it. Only thing you'll miss are the new races. That's how I've understood this whole grand scheme of things anyway.
 
 https://eu.battle.net/account/creation/wow/signup/

Vanilla WoW will sort you until level 60 and the new low-level stuff Cata introduced is folded into that as well.

No need to buy all the expansions up front. Just roll a Vanilla race and level to 60. If you still like it, grab the expansions in order. 

^ this