Branch in Barnes South West London, Branch in Chiswick, West London
Stash Yarns - Putney - newly opened by the girls who created the south west London S&B
Loop, Islington, North London
iKnit London - Vauxhall (one stop down from Waterloo station) they have a shop, a group and various other things
John Lewis - Good department store in Oxford street - great selection though not a good selection of needles.
Liberty London - horrific. Has totally gone downhill and the last time I was there the service was appalling and the selection left much to be desired.
Disclaimer: I'm not responsible for anything saucy or cheeky on those links... well, not much
My First Finished Project in 20 years!: Sirdar Foxy Scarf!