Wow, there are at least three small PC stores in the inner city alone here, can't believe those don't exist in the UK, maybe you just gotta look closer.
And of those two options given, I won't vote for either. The PC World PC has a graphicscard that's not even good enough for current games and the overclockers PC has a slower CPU. I'd say save your money for now, wait a bit, have a look at the options you have and then you can get a PC that will really last you for some years. You'll also have the money needed for it by that time.