Computer fairs are the best way to get hold of cheap computer hardware and software in London. Prices start at parity with mail-order and go down from there. Be prepared to pay in cash, though.
The emphasis is definitely on the latest hardware - don't expect to find older motherboards or processors here. On the other hand, some fairs have second-hand dealers as well.
Computer fairs generally happen on Saturdays, starting at about 10-11am - it's worth checking the time and turning up early.