HMV have some great offers hidden away in their Music From £2.99 offer. You have to do a lot of digging to find them but I noticed these cheap box sets at bargain prices:
This Is Reggae £3.99 - Brilliant 4 CD box set of classic reggae from the golden age of 1960 to 1975 - loads of ska/rocksteady/roots etc
The Message:The Story of Sugar Hill records £3.99 - 4 CD box set picking the best from the catalogue of the original hip hop label. Includes Grandmaster Flash, Sugar Hill Gang etc
Baby Bird - The Original Lo-Fi £2.99 - 6 CD box set of the band's original (and hard to find) albums recorded in a bedroom (before You're Gorgeous).
Uriah Heep - You Can't Keep a Good Band Down £3.99 - 7 CD box set with facsimiles of all their early 70s albums
Utopia - Last of the New Wave Riders - 7 CD box set of live recordings by Todd Rundgren's band
For vinyl fans, you can can get 3 LP box sets of the Complete UK Upsetter Singles (Vols 1, 2 & 4) for just £3.99. These are full of brilliant reggae produced by the legendary Lee Scratch Perry.