I'm sure you're right, it'll never happen, specifically because you couldn't fence it off.
A small confession though, I must have something missing as I don't at all mind paying to get in to places and have no urge to claim I have some ancient right to free access. Someone has to pay to upkeep places, why not we visitors? Seems fair. Who else, taxpayers? Why?
Anyhow, since it definitely ain't gonna happen it's not worth bothering about. BUT since the country is skint and these places DO need looking after, why not honesty boxes? Who would be so mean as to think five quid wasn't reasonable for a visit?