Certainly the John Lewis in Southampton had the red one. It may be worth ringing the John Lewis store to see (
www.johnlewis.com lists the London stores). Their on-line store is showing the red as sold out; I think they only carry the red and silver ones, and not the blue version.
The other possibility would be to try a Sony Centre; some at least do have them in stock.
www.sonycentres.co.uk details some shops you can ring to see if they have a blue one available.