The T-Mobile Vivacity is essentially the T-Mobile version of the Orange San Francisco II, and you can currently grab it on T-Mobile PAYG for £49.99. Good lord, it used to go for twice that.
T-Mobile Vivacity specs include a 3.5in display, 5MP rear camera, secondary camera on the front, and Android 2.3 (Gingerbread).
Minimum top-up of £10 required.
(Thanks to pablomalin at HUKD)