youre in the UK, so I can give you a few reference points to look at.
From now on you can use Time Machine, but it might also be worth looking at using Carbon Copy Cloner or Superduper on a regular basis, in which case, get one large external for the TM, and then a secondary smaller drive for CCC/SD.
depending on what machine youre using (MB? MBP? iMac?), and your usage, my recommendations would change.
Basically, if youve got FW800 on your machine, use it (file transfers are way in excess of USB2, making TM backups much faster). Even if you dont, I personally think that its worth looking to FW400 (in tandem with USB2) despite Apple making it redundant on their latest machines.
Anyway, im blathering a bit.
My ideal drive for the right here right now is
this one. Its big, its got USB2, FW800 (and an included FW400 cable) and eSATA, and its cost effective at £111.
If you dont want to stretch beyond the £100 mark, dont mind about the interface much, or dont think youll ever use 1TB of space, then for £65 you can get a
WD 500GB drive which only has FW400 instead of FW800, but still has eSATA. (you can get it with just USB2 for £60, but the £5 is well worth it in this instance).
i wont go on about a secondary drive, cos its not vital, and would only really be used in a dire situation, whilst TM should have you covered anyway.