If you've got the time to hunt for a bargain, then ebay and the second hand market is where you'll find them if you're lucky.
The £50 price mark is a tough one to stay under, but If it was me, and I had to buy a new one right away i'd consider the orange Sanfransico II from Orange...
San Francisco II from Orange in black | Pay as you go| Mobile phones | Orange
Its a decent phone and very hackable (if you're into that sort of thing)
ZTE Crescent - Crescent.MoDaCo.com - MoDaCo