There is a place called Fabrics Galore in Lavender Hill, Clapham I haven't been there but I have heard good things about it.The website isn't all that helpful as its being re-constructed and they've always said that their fabrics change so frequently they wouldn't be able to keep the website up to date.
52-52 Lavender Hill, SW11 5RH
There are others that come up in a search of
www.yell.co.uk (directory enquiries) so depending on where you will be staying in London you might find a closer one
hope you enjoy your visit here!