The GTX 660 is a bit quicker.
Zotac GTX 660: http://ncix.com/products/?sku=76364&vpn=ZT-60901-10M
tiny card, it will fit no matter what.
Powercolor Radeon HD 7850: http://www.newegg.com/Product/Product.aspx?Item=N82E16814131472
It is highly overclocked and is pretty short.
Sorry about the non UK links, but I do not know what good UK stores there are.
NCIX does offer international shipping though...