depends on what u like but i usualy check eukatech in coventgarden and the one under the skateshop in neilsyard also covent grd then you have one in camden under vinylbar for secondhand stuff soho is quite good

http://www.google.nl/search?q=cache:...ndon&hl=nl

http://www.google.nl/search?q=cache:...mden&hl=nl