Worth mentioning that with gpu prices being very high atm, and potential for current gen cpu prices to drop with Intel 12 series around the corner it might be worth holding off on any upgrade for a few months. It looks like gpu prices will remain high for at least another year, but may improve a bit, but might well be more potential to pick up cut-price new or second hand cpus. That said:
I think the 970 and i5 4690k are reasonably well matched so there isn't an obvious either/or to upgrade, but I think the 970 is probably slightly more likely to be limiting performance depending on the game, even assuming you game at 1080p.
If you could get a 3060 Ti FE for £370 (need to get the Telegram app and join the FE partalert channel to get notifications when stock comes in (roughly every 2 weeks I think) or you've got no chance) then that would be the best value option. Next price bracket up would be a 6600xt for £450ish (NB it's not as fast as a 3060 Ti though, so much worse deal - as are all brand partner cards). Both of those cards would be severely bottlenecked by your cpu, but any limitations due to the gpu would be gone.
If you could get a used 1080Ti or something then that would also be a great upgrade, but I think the used prices are a bit too high to make sense atm compared to getting a new card at that performance level.
AMD is the best option for me cpus atm, but they don't have anything available at the mid to low end of the market so for a new cpu you're looking at Intel again. Something like a 10400F, B560 mobo, 16gb ddr4 ram which would be £300ish. Going second has could be worth getting say a ryzen 1600 with new mobo and ram though (could be £200ish for that combo).
Could be worth dropping in a second hand i7 in to your current mobo to see if it improves things a bit though. Depends if you can get one for a good price.
