hmmm....... I'm not sure you'll get anything decent for that kinda money at overclockers to be honest; it does look like they're pretty expensive
Personally I'd advise against the dual-harddrive setup. The 250G drive will be slower then the 1TB drive and in the past I haven't found much use for separate drives unless you make the OS drive a really fast one (like a Raptor or SSD) but that'll set you back GBP300 for the SSD alone

Also; for the price of that second harddrive you can upgrade the 1TB to a 1.5 or 2 instead; even more bang for the buck.
Cheapest combination I can make at Overclockers of this all would be about GBP400 and include:
Patriot G Series 4GB (2x2GB) DDR3 PC3-10666C9 1333MHz Dual Channel (PGS34G1333ELK) £88.99
Microsoft Windows 7 Home Premium 64-Bit - OEM (GFC-00599) £84.99
AMD Phenom II X2 Dual Core 545 3.00GHz (Socket AM3) - Retail (means: including cooler) £69.99
Gigabyte GA-880GM-UD2H AMD 880G (Socket AM3) microATX DDR3 Motherboard £68.98
Hitachi Deskstar 7K1000.C 1TB SATA-II 32MB Cache - OEM (0F10383) £56.98
Coolermaster Elite 335 Case - Black £29.99
If you can find a supplier that has more decent prices then you should go for:
- Cheapest AM3 motherboard you can get. I personally like AsRock as they standardize better then most so less driver trouble (although Win7 solved a lot of those already). I have never had any complaints about their quality.
- Cheapest Phenom-II processor you can get. Just about any processor is fast enough for WoW as it leans heavily on RAM (and Cata on graphics card).
- Cheapest set of 2*2GB DDR3 memory
- A case to your liking
- Win7 Home Premium 64-bit (depending on how many PCs you want to install you could have a look at the "Family" edition; 3 licenses for about 1.5 times the price)
- A harddisk to fit your budget

The 2TB are best value at the moment so if you can spare the 100 quid then go for it.
Also; you could up your budget a bit by convincing Pogg that you're Dutch........