For those interested in flying in the vicinity of National Trust sites, I have just found the following - details of the Bye Laws that apply to NT properties. https://nt.global.ssl.fastly.net/documents/the-national-trust-byelaws-1965.pdf
These Bye Laws are quoted by NT as the substantiation for not permitting the use of drones on OR over NT properties. I’ve had a good look through the document and cannot find any reference to drone whatsoever. The nearest “hit” is the term “conveyance” either by land or air.
Anyway - what I believe is the most interesting section is “Penalties for Infringement of Byelaws - Para 26” It would appear that the maximum penalty for infringing their Bye Law is a maximum of £20 - compared to NT fees for an official permit, I would suggest the penalty is a bargain