The GUP japan smart drive is great as it silences as well as cools. There is a standard version for about £50 and a cooper version (not a typo) which is copper with extra cooling. Both take upto 15,000 RPM drives.
Slightly cheaper is the Scythe quiet drive for about £25.
For pure coolers there are:
Xilence Icebox Passive Hard Disk Cooler at about £20
Zalman ZM-2HC2 Hard Drive Heatpipe Cooler at about £19
Coolink Silent Savior, Quiet Hard Drive Cooler at about £10
You will have to search for them as I don't think OcUK sell them.