I don't really know if you can upgrade the entire computer for only £1000 since you need a CPU, motherboard, RAM, a GPU, and then probably a PSU.

Maybe this? https://uk.pcpartpicker.com/list/9xgJyX

That's just what I threw together in 2-3 minutes of looking at lists of components—there's no CPU cooler, I don't know much about the motherboard, and it's missing some other components that you said you were carrying over.