Based on personal experience.
Avoid PC World high street chain - miserable range of stock and expensive. I only use them in emergencies.
Overclockers
www.overclockers.co.uk are superb and cater exclusively for people who are serious about their PC components. Next day delivery except for large items like cases. Used to be able to visit their building in Stoke on Trent for collections - don't know if still possible. Excellent forums not unlike Anandtech. I have been using them since last century almost.
Komplett take 4 days to deliver. Tiny bit more expensive but allegedly excellent post sales support. Very much broader range of stock.