< £10: Don't do it!
£10 - £19:
Arctic Freezer 7 Pro (very quiet in low RPM mode, good cooling)
£20 - £29:
Thermalright SI-128 (can run passive, great cooling, hard to install though)
£30 - £34:
Thermaltake BigTyp 120 VX (reasonably quiet, superb cooling, extremely heavy for push pin)
Scythe Ninja Rev. B (can run passive, near silent, great cooling, very big, can fit any 120mm fan)
£35 - £39
Thermalright Ultra 120A (can run passive, very quiet/silent, superb cooling, can fit any 120mm fan doesn't come with one though)
Tuniq Tower 120 (massive cooler, comes with fan controller, reasonably quiet, immense cooling)
Noctua NH-U12F (extremely quiet, immense cooling, quite large, heavy)
£40+
Thermalright Ultra-120 Extreme (quiet, can fit any 120mm fan but doesn't come with one, huge cooler, expensive, best air cooler on the market)
Those are my pics and my notes about each one, hope that helps.