When looking for kitting yarn I stumbled on I Knit London a year ago or so. It's hands down the most fun place to go and buy your wool or whatever you need.
Friendly and helpful (once you've been in a few times) and they run a knitting group that you can join. One of London's most obscure - and best - shops!!
I Knit: a club, shop and sanctuary for knitters
We have a shop full of yarn and other goodies where we host a twice-weekly knitting group. We host crafty events all year round, inclucing the I Knit Weekender, a knitting and crochet extravaganza including market, classes and exhibitions!