I'd say a RTX 3060 TI at RRP (£370), if you can stretch to this amount. You'd need to set up a stock alert service like part alert to actually buy one.
You could wait for the release of the RTX 3060 this month, which has an RRP of £299. Stock might be more plentiful than people are suggesting, since GPU miners will likely avoid this graphics card (it's hash rate is limited), at least for a little while. It's a lower end Ampere GPU, so it *should* have a higher production yield, also, it uses GDDR6 VRAM, which is thought to become available in greater supply soon.
If you don't mind waiting, next month should be when the RX 6700 and 6700 XT launch. Hopefully, the RX 6700 reference model will be sold for ~£300 on AMD's website.
Any of these graphics cards would be a huge upgrade from a HD 7970.
Personally, I'd go with a Nvidia GPU this time around, because of the launch performance benefits DLSS offers in some games, but whatever's affordable...