The usual suspects...bay, gum tree, FB. Ofc you'll need to be on the watch for scams etc.
Shame you don't have members market access. Could definitely do a very reasonable gtx 1070 or even a gtx 1080 build on that kind of budget.
The best would be to nab a new 3060ti FE and build around it. Doable for £650. Ofc you'll have to be quick when stocks drop (goes within minutes).
what's members market access? I suppose a reward for being a good OC buyer?