MUSIC & TAPE EXCHANGE, 95 Berwick Street, London, W1V 3PP, 020-7434-2939
Sister store to the Notting Hill collective, has a decent selection of all styles of music from prog rock to techno, with a bargain basement downstairs which, over the years, has yielded many gems, including mint copies of Gina X's No GDM and Chas Jankel's 3,000,000 Synths in the past couple of years.
website
The above have many branches but as an ex employee i know that both the ones in Camden town get the best stock and have excellent basements.