Your best bet to get one for as cheap as possible is to buy an internal hard drive and then just buy a caddy for it, Will work out alot cheaper, still your going to have to be pretty lucky to get a 1tb for £50 or under.
If your willing to spend a bit more. I got mine from Comet in the UK, its a 1.5tb and cost me £79.99, was a lucky one to be honest and I got it just before Christmas