I'd start by finding a cpu that you want. You say your interested in an i5 3570k. That seems very definite so start there. Then find your MB based on your planned usage. Are you going to want to overlclock? Are you planning on running crossfire/SlI? Consider this for ram too.
I realize that newegg.com does not ship to the UK but you can still use them as a resource. I would recommend looking up your cpu and considering there recommendations. Read their reviews and use several sources. Be prepared for the worst rated comments to happen to you and consider how you would handle that. Also, I would highly recommend and SSDboot/HDD storage solution. There are lots of SSD deals out there now.