I think I read a long time ago about a convention that anything in the royal collection from prior to 1901 was state property and anything after was private. I’m not sure I believe it since the royal collection website doesn’t differentiate.

The royal jewels do have Crown Jewels/Jewels of The Crown and private. There have been issues over the status of the duchies of Lancaster and Cornwall and it has been raised in parliament, where royal spokesmen says its private when it’s a state matter, and public when it’s a private issue.

A while ago there was an interesting report published about the Grand Ducal family of Luxembourg which has pieces of information about the Belgian, Dutch and Swedish Royal house (possibly Danish too) which is clear that publicly paid officials like secretaries and equerries should not perform private roles. In the UK staff are not separate like this. This attitude could explain why it is so hard to understand the ownership structure of them.

Another important thing to remember is some rules the Queen follows are from the beginning of her reign and maybe are conventions that are not real but are unquestioned due to QEII’s age. They might get clarified in the future. I doubt that future monarchs will be treated so well by governments, though they are embarrassingly servile. The Queen is treated a bit like the late King of Thailand.