At a little under 50 quid, there's a Hitachi X-Mobile 500 GB drive (which comes with backup s/w) and works with Win 7, Win XP, and earlier versions...
Comet have them at about 47 quid (can order online and reserve so you just visit the local store to collect). John Lewis also have them, with a 2 year warranty (not sure if that is default, but may be worth having).
It is designed as a portable so no mains transformer. Weighs under 750 g where some weigh three or four times that.
There are other portables closer to 40 quid, storing 250 to 320 GB but this doesn't cost a lot more, comes with software to allow for scheduled backups, and has been reduced from around 75 quid in July.
Alternatively, depending on the PC, it might be a cheaper option to see if someone (friend / work collegue) has enough PC knowledge to plug in a second drive.
You can buy anything from 80 GB up for low amounts of cash (get something off Ebay, or use google.co.uk and see if it gives you the 'shopping'option (so you can search for UK sites and see scores against various retailers if you prefer brand new).
Online stores I would recommend for low cost hard drives are scan.co.uk, svp.co.uk and ebuyer.com, if you must buy new (there might be some deals from PCWorld but generally I think they and Maplins are more costly).