A GPU upgrade is going to be your best option for higher FPS, so that's where I'd start. CPU is starting to show it's age, could do with an upgrade, but you won't get a CPU/Mobo/RAM upgrade as well as a GPU for £400, at least not anything that's worth upgrading to.
You could maybe grab a used i7-4770k/4790k or something to get a little extra life out of your current socket but to be honest, at this point, I'd just hold off and keep the i5 until you can do a full socket upgrade.
So, for GPU, if you don't mind waiting a bit, then I'd probably be on the lookout for AMD and NVidia's upcoming £400ish GPU's. So, like an RTX 3060(Ti) or RX 6700(XT). No official announcements of these yet though, so no idea how long they'll be. Currently, you would be looking at an RTX 2060 Super in the £400 class, but that pricing is too close to RTX 3070/RX 6800 to make sense since the 2060 Super is a significantly slower GPU.
The RTX 3060(Ti)/RX 6700(XT) that are likely to replace the 2060 Super are probably going to offer around RTX 2070 Super/2080 performance I would imagine.