if its an absolute budget of £125 and you want brand new then get an hd 6850 from HIS, Asus, XFX, there all about that price.
As for what Nevakonaza posted, they would be much better options and i presume he thought you would be able to stretch a little. You will see a lot better performance with the ones posted above.
If you cannot stretch the budget 6850 imo would be your best bet.