Swings and roundabouts mate. 970 or 980 is the way to go if you want Nvidia. 970 has VRAM issues. 3.5GB at the stated speed and then 0.5GB and considerably less speed nstead of the stated 4GB, and cost 280 upwards give or take a few pounds. 980s are the top of the range, start 400 pounds and upwards of 600. Unless you want a Titan.....
AMD 280/280X will full into your price bracket nicely and provide good performance. Close to that of the 970 if you get a 290/290X which are about 250ish!
Might be worth just browsing through some of the threads in the graphics card section to see personal thoughts and experiences of people that own the cards.
What resolution do you use btw?