Just about every 3.5 inch external drive enclosure I've used or heard about had an accessory power brick that required a wall outlet plug. If your intended use is with a laptop, and you need to use it in "mobile" mode, then you'll have to step down to drives with the 2.5 inch or 1.8 inch form factor. Many of them can be powered from the USB port, and a few offer built-in batteries (ex: http://www.newegg.com/Product/Produc...2E16822101034), but don't expect especially terrific battery life. Keep in mind that these smaller form factors don't offer anywhere near the same capacity or performance - that's sacrificed for portability.
Of the selection currently available at NewEgg (yes, I know they don't ship to the U.K.), this one (http://www.newegg.com/Product/Produc...82E16817392009) seems to be the best enclosure with both USB2.0 and eSATA for mobile use. With eSATA, it needs to either have an external power brick (not included) or have the USB cable hooked up (for power only). With USB hooked up, it draws power from that port.
Looks like capacities in the 2.5 inch drives peak at 160 gigs, with offerings from Seagate, Fujitsu, Hitachi, and Toshiba. Seagate offers both 5400 and 7200 rpm models, with the faster version having 16mb cache (at a higher price, I'm sure). Links here: http://www.seagate.com/docs/pdf/mark...tus_7200_2.pdf
http://www.seagate.com/docs/pdf/mark...s_5400_fde.pdf