Theres a new one in Covent Garden, cant remember the name, but it`s hugely expensive as you`d expect in that location. Not been in there myself, but I`ve got to pop to Covent Garden in the week, so was going to take a look-see. EDIT; it`s V-Revolution, they have a website too.
Other than that, nowt other than the cig-a-like brigade in the shopping centres. Though there`s a stall in the Treaty Centre in Hounslow which sells cig-a-likes, but he also stocks quite a few bottles of ejuice and Ego batteries, nice guy, will let you sample some flavours.
UK E Cig store have an address in Soho, but I don`t think they have a store there, just an office.