People who own the cars really can do whatever they like, but I do have some feelings about it.
I'm an "it depends on situation" type when it comes to converting classic cars like that. If you have a car thats already got a bad (or missing) drivetrain, or some other problems that make it really impractical to save in its normal configuration, than it seems more reasonable to me than tearing apart an otherwise sound example.
Other possibility is they could be building their cars on Motor Heritige bodyshell foundations, not retrofitting existing classics. That would just be flat out cool in my opinion, plus giving a lifeline to British Motor Heritage in the form of extra sales. I'm curious to know more about their product....