What Flexxar said is totally right, graphics might be the thing to invest in if you are gaming.
£200 is a slightly awkward budget for trying to upgrade CPU because you'll definitely need a new motherboard+ram, PSU is questionable too. It's also worth assuming that if you have a good quad core already, you probably don't want to step down to a dual core. This is where you hit price problems, as you can get a solid quad core AMD setup for that price but you will struggle on Intel.
If you really want to get a solid performance upgrade and are sure that you are CPU limited, I would personally try to stretch the budget to say £240-250 at which point you can afford a 3rd gen i5 Processor, a cheaper motherboard (H77 probably) and some ram. In theory that wouldn't require a new PSU.