Anyone know the best way? Rent a car maybe? I've always wanted an excuse to rent a right-hand-drive car, but if it's substantially slower than taking the train I won't bother.
It depends where you start from. If you're in central London, it's a nightmare getting out of the city and onto the motorway, especially anywhere between about 4pm to 7pm. Edit: I can't stress enough how *&^%$£" awful London is during rush hour.
If you start from Heathrow it's much easier driving as you're on the right side of the city and don't have to travel in to the city just to get the train out again.