I've just picked up a Samsung U600 mobile from the Carphone Warehouse (Newcastle) for £49.95 on O2. They were the same price for other networks, but unless you already have a sim card for the network you want, you'll have to top up by £10.
I couldn't see this on the website, so it may be in-store only.
The same phone was "reduced" to £69.99 in Phones 4U.
And it came with a 2Gb Micro SD card for £6.99!
Specification: Samsung U600
3.2 mega-pixel camera with flash, 4 x digital zoom & autofocus
Video camera with MPEG4 format
TFT display with 320 x 240 pixels & 262,144 colours
MP3, eACC+, ACC+ & AAC music player with digital power-amp
MP3 & 64 voice polyphonic ringtones
Built in handsfree speaker phone
SMS, Email (POP3, IMAP4, SMTP, TLS, SSL) & MMS messaging
PDF, MS Office & HTML document viewer
Mobile printing using Bluetooth® PicBridge
Built in java games
60 Megabytes of memeory with Micro SD memory card slot for up to 2GB
Bluetooth 2.0,TV out & USB 2.0 connectivity
EDGE technology with WAP 2.0 & GPRS Class 10
Talktime 4 hrs
Battery standby 270 hrs
Size 11 x 49 x 103 mm
Weight 81 grams