I just took a quick look at Maplin.co.uk, Comet.co.uk and Ebuyer.com (I'd generally expect PC World to cost a bit more in their store than online)
It depends a lot on how much you need to store.
Prices are from about 35 quid for 160 GB (Ebuyer QuickFind Code: 149252 ) to whatever you want to pay.
I remember seeing some which have software and claim it's a simple "press one button" on the drive to force a backup, but as stated earlier, it doesn't need a lot to DIY:-
Sumvision external case - QuickFind Code: 136250
Then choose an IDE/PATA (not SATA) Hard Drive with capacity depending on your budget.
If you DIY you could easily put different drives into use, eg keep a copy of all downloaded software on one drive, keep music backups on another, etc, etc. Some of the enclosures are big enough to put CD/DVD in, and those are really easy to use with different HDs.
I have about a dozen different HDs for backups - 40 GB, 80 GB, 160 GB and a 500 GB. Only plug in when backing up, so any virus etc cannot corrupt the external drive.