As said Hein Gericke are at Kenton and Hanger lane, there is an Infinity at Hanger Lane as well, if you have transport then there is a very large J and S Accessories on the A41 on the outskirts of london at Junction with junction 5 M1, they have a very large range, there is a large ish motorcycle dealers HGB at Ruislip Manor (30 seconds from tube station) they have a fair clothing department.
A bit further afield there is a great helmet and clothing shop ins Stevenage (just north of london on A1) called Bikestop they have a very good range of "better" clothing and helmets.
Bit further out west in slough you have George Whites a massive massive store, they sell everying thing from the cheapest crap to £1000+ rucka stuff.
If your interested in Klim then the only dealer round here is The KTM Center in Hemel Hempstead, they currently have the whole range in stock I think including the adventure suit.
Harpers of forest gate in east london are very good and suppliers of Revit gear
If you want halversons then Skins Cafe 71 High Street Brackley NN13 8BW its a bit out of london but only 45 minutes north or so and thats all they sell.
If you want to go east further then Hideout Leather in safron Walden in essex again not that far from london they have a massive range.
With all of these I would have a look at their on line web pages first and possibly ring them if you have anything particular in mind.
hope that helps.
