There are several shops down Berwick Street, which runs parallel to the top part of Regent Street (Where Liberty's is situated) Another good place to go is the Goldhawk Road, also the name of a tube stop on the Hammersmith & City line. Come out of the station and turn right and on both sides of the road are loads of fabric stores - granted they are more clothing fabrics, but the prices are good & the fabrics are fabulous.
When visiting London I would heartily recommend getting yourself an 'Oyster card' for the transport system, this is a prepaid debit card that you just wave at the machines as you get on & off the tubes & buses so you don't have to worry about getting the right tickets (there might also be a bit of a discount on the individual prices too)
Just googled quilt shops in London and got these
http://cargocultcraft.com/knowledge/lonely-crafters-guide-to-london/
http://planetpatchwork.com/travel/london.htm
http://blog.visitlondon.com/2010/04/fabric-shops-in-london/
http://www.beyond-fabrics.co.uk/
The first 3 are lists of recommendations, but depending on when they were written double check the shops are still there.
All in all it is very limited for pure patchworking/quilting shops and nigh on non-exhistant in the centre of London.
