I wanted to buy from the Uk though
Ack! Sorry for being ignorant to that.
Brands that I would recommend (from experience) are Corsair, Kingston, and Crucial.
You can also head to http://www.crucial.com and use their memory configurator to see which memory is right for your iMac.
You can also check out RAMseeker for sites that will sell the memory for your Mac at a good price. Don't know if they cater to UK customers, though.