Oops, correction, it's an older M3600 and yes, this still has a Quadro (CUDA-compliant) card in it. The base M3800 is about £1,400, but fully spec'd, maybe £2,500. Well worth it, and the M3800's faster than the top MacBook Pro.
The Quadro card in the M3600/3800 and the bigger 17" models is fast! I've been editing 4K raw material from Sony F55 on it for a short film recently and it's absolutely fine. Ain't got time for gaming, but I'd suggest running a 2nd machine for that anyway. Games playback generally requires a different way of setting up for GPU anyway to get the most out of it, and that generally quite different from workstation-class edit machines.