Here's a run-down of the chief UK 2008 sf and fantasy conventions which focus on the written genre. This list is by no means complete...
21-24 March - Orbital 2008 - Radisson Edwardian Hotel, Heathrow
26 April - alt.fiction - Assembly Rooms, Derby
19-21 September - Fantasycon - Britannia Hotel, Nottingham
11-12 October - NewCon 4 - The Fishmarket, Northampton
14-16 November - Novacon 38 - Quality Hotel, Bentley, nr Walsall