Hopefully this will help someone....
Spotted this in Smyths the other day - also shows on their website but you can only purchase this in store. It's the cheapest I've seen - next cheapest according to find-dvd.co.uk for the Wii is £87.49 from Powerplay - Xbox 360 says £79.97 from PC World but we all know what that means! (Next cheapest is £97.86 from Amazon) and PS2/PS3 is £99.99 from Amazon. :D Here's the blurb from the site...
ROCK BAND Band in a Box for PS3, X360, PS2 & Wii
Was: £109.99 Now: £79.99
Includes Guitar, Drums and Microphone Requires Rockband game (sold seperately). All Rockband items can also be bought individually. Available on PS3, PS2, X360 and Wii. Check instore for full details and special offers.
The website doesn't say when it's on offer until and there's a stock checker on the site if you want to see if your local has any in stock ;)