Personally, I would upgrade the CPU instead of the GPU. Your card is more reliable for the future than your CPU is. Although, if you were to upgrade your CPU, I see that someone suggested a 7700k, and even though I agree that would be a good choice, those have been discontinued, so if you were to find one, you'd probably end up buying it for the same price as an 8700k which is like £350. I would recommend you get an i7-8700 which would be like £290. But, once again, you would have to upgrade the MOBO to one that supports the new CPU. I'm not very 'pro AMD' but that's just because I don't invest time to learn about them. But if you were to go AMD, you would also have to buy another MOBO.
Bottom line:
If you upgrade your CPU, AMD or otherwise, you're going to have to get another motherboard, this will probably cost you more, but you will be future-proofed WHEREAS
if you upgrade your GPU to something like a RTX 2070 you're going to notice your PC being slow in other areas. But yes, if you want results now, an RTX 2070 would be the route to go down.
PERSONALLY, I'd get a new CPU, but that's just me. Hope this helped.