Chronicles of Riddick: Assault on Dark Athena, PC - £23.49 delivered
Alone in the Dark, Xbox 360 - £6.99 delivered
Eternal Sonata, Xbox 360 - £5.99 delivered
Pikmin (New Play Control), Wii - £19.99 delivered
Ratchet & Clank: Size Matters, PSP - £7.20 delivered
Just got a tip that GHWT with guitar is forty quid in Morrison's stores right now. The store checked (Congleton) only had the PS3 version available, but apparently other stores also have the 360 version.
A quick check on eBay shows the game itself reselling for just over £20, so if you're looking for just an extra guitar for either game this is probably your best bet.
Each version of the guitar is compatible with every other guitar-based music game for the same system.
1600 Microsoft Points - £16.63 delivered
This is intended specifically for the Lost and Damned expansion for GTA IV, but you can use the points for anything. The 2100 card one post below is better value but if you won't use the extra 500 on anything then this is your best bet.