The only "Auto Train" in the USA runs from Virginia to Florida. About 800 miles and 12 hours to drive. It is well patronised by "snowbirds" (seasonal residents).Driving is more attractive to many people who have large amounts of luggage, children, or both to transport. The idea that you can get into your car in your own driveway and get out at again at your destination point is attractive, and far more so than having to get tot he station, check your bag, travel to wherever you're going, retrieve your luggage and then get to your hotel, B&B, cottage or whatever.
It's another niche market. Attractive for a few, yes, but probably not at all viable.
Anything less distance/time would not be attractive. Anything much further risks potential clients having to double back to reach their destination. Few are OK doing that for any distance.
So if, big if, there would be a case for a UK Motorail it might be from somewhere around Reading to somewhere in south central France (near an autoroute interchange).
But British seasonal residents (summer in Britain and winter in the sun) tend to Spain, Italy or Greece. Folks don't mind overnight on the train followed by perhaps up to six hours driving to reach the final destination.
The simple fact is though, Britain is not geared up for seasonal residency and doesn't make it easy for people who want to live that way. Unlike Florida for example.