At Alton Towers today they had a sale on of Annual Passes. 25% off. This deal is valid until the end of next week and is only available from the park.
The Standard pass is reduced to £90 and the premium pass is down to £120
The standard pass has some restrictions on visiting dates to Merlin attractions and doesn't include car parking. The premium pass has no restrictions and includes parking.
Check the Merlin website for details of each pass http://www.merlinannualpass.co.uk
Passes give you access to a lot of uk attractions including Alton Towers, Thorpe Park, Chessington, London Eye, Dungeons, Tussauds, Warwick castle, Sealife etc. See website for full list.