For that kind of budget I think you may struggle to see any real improvement over the 970. For the £200 mark you are looking at a 1060 which a lot of benchmarks puts on a fairly close par.
There's the AMD option of an RX580, but you will likely only see a 10% improvement in performance.
If you can stretch the budget to the £350 ish area then you could get a GTX 1070 which would give a performance boost, but that's obviously a fair amount more money than you want to spend.
For my two pence, I would save more pennies and get a 1070