Take the train. It's the cheaper option, and the extra time on board is usually more relaxing than driving it.
Tickets start at around £120 compared to roughly £169 in fuel and tolls — a saving of about £50. The journey takes 7h 20m versus 7h 15m by car, so you'll arrive a bit later, but you can work, read, or sleep along the way rather than spending the time behind the wheel. You'll also save the hassle of parking at the other end.
Driving is worth it if you're on a tight schedule, travelling as a group, or want to stop somewhere en route.