Hey,
Russell Square, is that near Euston in WC? If so, take the Picadilly line south to Holborn, then switch to the Central line to Tottenham Court Road and exit. You'll be on Oxford Street, and there are TONS of great places to get second-hand and new stuff.
The best places are the Music and Video Exchange chain, there are about 6 in London proper, here is a link to their sites, you can see pictures of all the vinyls they have:
http://www.mveshops.co.uk/info/locations.htm
When I lived there I went to these shops every weekend for 6 months and always left with something good. Mind you, I was looking for techno, but I remember that they had a pretty decent hip hop collection, you have to know what you're looking for.
Another is Restless Records, it's on Warwick street, which is between Oxford Circus and Tottenham Court Road in Soho. Last time I was there they had split the shop into two big rooms, there was TONS of vinyl.
On Camden Road there were a few new and used shops, take the Northern Line tube to Camden Town and look for all the street markets, they're in there.
Of course, the big HMV and Virgin on Oxford Road will not disappoint, and there's a big Tower Records at Picadilly Circus. If you stick in the Soho section of London (you're right next to Soho) you'll find all sorts of neat shops, new and used.
Best of luck!
Nick