https://uk.************.com/p/PHcJ4D
This would roughly be what you can build with 800 Pounds. Of course most of the parts are switchable for other brands and stuff this is just so you can see what the total would come out to.
GTX 970 is about the same as the R9 390. R9 390 would only be useful if you're really trying to drive 4K or multiple monitors.