Dunno about london - but where I am theres a few smoke shops that sell seeds...just find one nearby which does loads of smoking stuff (pipes, bongs, hundereds of different rolling papers, lighters, etc) They'll have seeds if you ask - I guarantee it...or else try
www.paradise-seeds.com ...they take cash by post and ship in a week from amsterdam if your a bit wary of the old bill - remember, OWNING seeds isn't illegal in England...but GERMINATING/GROWING them is! (which is just stupid really if you think about it...smoke shops are safe coz they can say "Yeah, we sell seeds - its not our fault if people grow them, coz once they've got them they can do what they want with them...we only sell them")