For £500 you're not going to get a new platform + GPU so I'd suggest an upgrade to your existing rig. I'm going to assume your PSU and RAM don't need upgraded but let us know what they are to confirm if that's true or not.
Check your motherboard supports the 5000 series CPUs (get your BIOS updated) and pick up a 5600/5600X or 5700X3D, the latter gives the best gaming performance but is more expensive, the 5600 is still waaay faster than her existing CPU.
That leaves you with £300-£400 for a GPU. Obviously AMDs new cards are due any day now and although you likely won't be getting one of them for that budget it might be worth holding out to see what happens to the market (?!)
Personally if your PSU is good enough I'd try to find a 7700XT/7800XT if you can, as a minimum somerthing like a 6700XT/6750XT will fit that budget and have great performance. You also have the option of the Intel B580 but with a lower end CPU it can struggle and it's about as fast as the 6750XT anyway.