I used to use the train a lot, and I still check the fares to see whether I can avoid hiring a car. Unfortunately, unless you get lucky, or are booking 12 weeks in advance, it’s just too expensive. Heathrow to Dover and back was coming in at £150, whereas a car for the weekend was £65 – stupid though as the car will be sitting on the drive for the weekend!
Even if the car hire works out a little more, it’s usually worth it for the extra flexibility.
Shame really, as when it works it’s a really nice way to travel. Dover to Leeds over Christmas one year, stopping at the champagne bar at Charing Cross en route, still ranks as one of my favourite trips…