The difference between is mostly just in the naming. AMD rebadged the lower 7*** series for some of the new range and are pretty much the same cards other than not needing to use display port for 3 screens.
£100 is at that point where you don't get a lot for the money but if you add a little more you get something much better, £130 for an r9 270 is probably where you'd want to go for a lower end gaming card.
If it's not flexible then perhaps go for a b-grade from the last season cards, something like this 660 would be decent enough
http://www.overclockers.co.uk/showpr...=595&catid=683 or either keep an eye out on daily deals at the various sites or go second hand.