Depends on your PSU, if it's at least 450 watts or so from a good manufacturer (Seasonic, Corsair, Coolermaster etc), then you could go with a 6770.
http://www.dabs.com/products/xfx-ati-radeon-6770-hd-850mhz-1gb-pci-express-hdmi-7NQV.html?q=6770&src=16
That's a good one for the price. Uses little power, and comes with 2 free games. XFX also overclock well, and they have really good customer support. Their warranty even covers overclocking damage, so that's a real bonus too.
Imo, it's pretty much the best card in that price range, as AMD have generally good price/performance ratio, and the 6770 should be decent if you plan on playing games a lower resolutions.
EDIT: Use the quick edit button (Next to edit). If you plan on going for the £120 mark, I'd recommened a 6850. It almost matches a 560, although a little slower. If your CPU is up to it, you'll be able to play games fine on higher resolutions, and the card still draws little power (127W under load), so can run on even a Corsair CX430w.