If your purpose is gaming, then buy the strongest graphics card you feel comfortable paying for.
If you game at 1080P, then a GTX970 around £270 will be well within your budget at around and will play most anything well. If you game at 1440P, then GTX980 would be better.
If you have aspirations of gaming at 4k resolution, you are looking at GTX980ti.
Your budget will buy a GTX980, around £400 but that card is in a strange place in the marketplace.
It is stronger than a GTX970, but by not as much as the price differential.
The very best graphics card will be the GTX980ti which is around £550.
Then, there might be the question of how strong your psu needs to be for these cards.
You are looking at 500-600w.
Here is a chart:
http://www.realhardtechx.com/index_archivos/Page362.htm