I got into that "Where does the Road Traffic Act apply?" thing at work about 10 years ago. It's a mess (like most laws).
If the public "could" drive there, it's public highway and the act applies. Even if there are big "private - no entry" signs. Unless there's a closed gate and a guard, it's public highway. A "pay £1" gate doesn't stop it being public highway.
But the landowner can still charge you for access to it.
So, yes, they *can* have it both ways, Don't blame me - I left all this nonsense for the sane world I now live and work in.