I'm not 100% sure, because I have never entered the shop, but karenearnold is correct: there is an oddity of an off licence at the end of old compton st (walk up wardour street on the right hand side from shaftesbury avenue, turn right onto old compton street and the shop will greet you very early on the left hand side) - there is a very striking picture in the window of a man looking seriously worse for wear, presumably due to a heavy session of said toxin! It looks Van Gogh-ish but I have never looked close enough to be sure. I would think you had a pretty good chance of finding absinthe here.