Maplin have a WD 250GB see here for more info.
Seagate hard discs are highly regarded, so one of their external drives should also be good.
Do you deifnately need an external drive? Internal ones are cheaper and dead easy to fit.
Can you get external hard drives that are powered by USB, rather than needing a power adaptor?
Get up to £25 this Saturday
What MoneySaving things can you do now to get ready?
Including already-reduced outlet stock