Take the train. It's meaningfully faster than driving, and the time saved usually justifies the price.
You'll arrive about 8m sooner (6h 48m by train vs 6h 56m by car). Tickets start at around £148 compared to roughly £102 in fuel and tolls — so you're paying a bit more, but getting hours of your day back, a city-centre arrival, and the freedom to work or relax instead of drive.
Driving makes more sense if you're travelling as a group (where fuel costs split several ways), want to stop along the way, or need a car once you arrive.