GeForce
"not sure if you can get HPC programs to recognise and use a GeForce GPU without a lot of messing around"
As long as you can get CUDA drivers for it I doubt it'd matter (the software would use the CUDA API unless some fool put a nonsense arbitrary limit in the software in which is highly unlikely given there are supercomputers that use actual graphics cards out there).
By the way you're talking about a 4500 GBP versus an 800 quid card - for the sake of a bit of RAM if you don't need it you have a massive price:flops advantage.