....this could take a while...
OK, good start with the notting hill gate exchange, also in notting hill are Intoxica and Honest jons (both on portobello road, honest jons is a LONG walk down the portobello from the notting hill gate tube but well worth it)
Camden has a few good market stalls and another good exchange shop on camden high street, theres also a good shop just off camden high street that sells some good 45s but i forget the name (london strutters help!)
Oxford Street area- has berwick street and poland street coming off it, has revival records (previously wreckless), sounds of the universe (soul jazz's store) another MVE exchange shop, vinyl junkies (can be good selection but kinda always on an overpricing tip)
Islington & Angel- haggle is a good place but the owner is crazy as F*ck, theres also flashback in that area which is good but some of their wall pieces get overpriced.
Theres much more but these are the places i aim to hit if i've only got a day in london, if you're really short on time i'd say just aim for the trade stores, you can find some gold in there for next to nothing.