Best gpu for video editing?

Solution
For that price range the best are
NVIDIA
760 < 770
AMD
R9 280X < R9 290

roughly in order of power

760(200 pounds) < R9 280X(230) < 770 (250) < R9 290 (300) next jump is about 380 pounds
Unfortunately right now Nvidia's cards are better for editing due to the use of CUDA which most programs use and are optimized for esp. professional however a good chunk of programs now use OpenCl as a backup which can be used on AMD cards however mantle seems to have the most effect on systems with low powered CPU's so you may not miss it.

So the question left is what are the rest of your system specs and which program do you plan on using?

Either way though the answer is

NVIDIA
AMD
 


In that case I would get the Nvidia card since your CPU is strong enough that Mantle will have little effect on your FPS (somewhere between 3-5) Your program of choice would really cement the decision though.
 


Well all professional programs at the moment support CUDA (NVIDIA only) and is usually faster and more mature and as such most none professional programs are similar. OpenCL which is supported on AMD and NVIDIA is more recently implemented and less robust. So if you want to do serious editing the NVIDIA card is the better option just because the state of the market favors CUDA at the moment.