Any shop on Tottenham Court road will be able to source you one dude.
Make sure however you tell them you live in London. They are notorious for ripping off tourists if they spot you...also they ALL talk to each other. One shop at one end is normally owned by the same bloke as one further along. So trying to bat them off against each other is pretty hard.
BUT do bargain and offer cash ;)
They will probably source you one fairly quickly. I remember when the XBOX360 came out. On launch day nobody had one, but for £40.00 extra you could have as many as you wanted from one of the shops on that strip
Good luck my friend, and enjoy the city. Its pretty neat