Okay J& S in New malden have a huge selection of Bike helmets, Frontiers in Wimbledon Chase have got a few, Dobles in Croydon have loads, Infinity in Guildford have loads, infinerty in farnborough (formerly Motorcycle city) have loads. Otherwise look in the yellow pages.

I to would suggest trying one on getting the right size and then looking around. Try in the back of MCN (motorcycle news paper in newsagents) as firms down in the south west and distant places do them a lot cheaper mail order.

Good luck.