There aren't as many as you might think! I visited back in August and did a little diggin' online beforehand. Shops have emplyee's to pay and high rent in London so their prices border on ridiculous in the case of the new Covent Garden
shop.
I found a great store in Wembley called UKecigstore. They wholesale worldwide and have a great
juice sampling area and hardware at reasonable prices. Just get off the tube at Wembley Park walk 5mins and you're there, street is called Rutherford Way I think right next to the Police station!