For that budget you can get an enormous upgrade, but whether or not it'll be worth it depends on your use case. What games, what res, use of nvidia proprietary tech such as DLSS/RTX etc etc.
Used? The best you can get at the moment is likely a 5700XT or something around a 1080ti/2070 super on the MM.
New? I wouldn't even think about it right now to be honest.
:edit: ah! So I reread and saw you said under £250-300, my recommendations are at the high end of that. You're looking more at a 1080/2060 super/2070 or 5700 level for under £250 really. It's worth assessing your requirement as you will likely still get a really big upgrade from something much cheaper.