Many USA suppliers will not ship to the UK. Even if they do, you will get stung for all sorts of charges/duties etc and it isn't worth the hassle. I won't be much, if any, cheaper after all this.
For UK suppliers, look in the Links section of this forum and also among the posts to this column for opinions. Personally I use dabs, Scan (esp. their daily specials), e-buyer, Watford (sometimes), Overclockers, Microland (sometimes).