WD 500 GB My Passport Go Portable SSD - Blue £49.97 / Yellow £49.98 @ Amazon
Decent prices on these. It's worth pointing out the USB cable is hardwired (watch the video).
Drop-resistant up to 2 meters, with a rubber protective bumper to endure bumps and shakes even while the drive is plugged in
Pocket-sized drive has a built-in cable for convenient portability
2.5x faster than most portable hard drives with up to 400MB/s performance
Solid state drive with no moving parts, purposefully designed and manufactured by WD for high reliability and dependability
Works with PC and Mac computers, with included automatic backup software for Windows and compatibility for Time Machine (reformatting required)
* Please note that all prices are correct at the time of posting. However prices can vary quickly and significantly without warning.