The largest online resource dedicated to BDSM in the UK is Informed Consent, which hosts BDSM discussion boards, personal ads, and listings of fetish shops, BDSM events etc.
In the London area, London Fetish Scene has a magazine style format and reports on events in the capital, especially the fetish clubbing scene. LFS runs the Wipipedia, an encyclopedia of Fetish and SM (which also appears on Informed Consent as the BDSM Encyclopervia.)
The Spanner Trust and Backlash are the main groups campaigning for BDSM rights in the UK, and the website BDSM Rights lists additional UK and international groups.
Specialist UK-based groups and sites also exist, such as the Slave Register (which has it's own Ownership and Master/slave Wiki with topics like slave training) and The Mistress Index (for professional domination). Special interest groups are usually included in the listings on Informed Consent.
Tanos, webmaster AT bdsm.org.uk