Easy. Multiply £80 by 2 and wala! £160
On the serious note, the OP clearly stated 'must be less than £80', so people would think any (much) higher price is not an option, however now that the OP has clarify £80 is not the absolute budget, then that open to lots more options.
Between £140 and £180, the GTX460 768MB and the GTX460 1GB is what you should consider. Unless you got some personal preference of ATI over Nvidia, I wouldn't recommend the 5830 over the GTX460 768MB considering that both are similiarly priced, but the GTX460 768MB is faster in most cases:
http://www.anandtech.com/bench/Product/171?vs=156
At £200 there's the 5850, then at £220 there's GTX470. Both are decent cards, but the GTX470 is faster...at the cost of higher power consumption, higher temp and nosier:
http://www.anandtech.com/bench/Product/164?vs=160
ok I barely understood some of that but which 460 should I get