are also good for online, not sure if they will ship to you?
In London Run and Become
probably have the best selection. They're conveniently located very close to Victoria Station. RunnersNeed stock some stuff, but it's far from a wide choice and tends to vary between stores. The Sweatshop store on Trump Street also used to have a good selection of stuff, although I've not been in lately so not sure if that's still the case...