Is there any regulation / ruling that states my en-suite toilet must
have be behind a door?

I'm afraid not, as I understand it. It may not be in the same room as a food
preparation area. Its associated hand basin may not been in the same room as
a food preparation area. However, I believe a toilet and its basin may be
installed in any other room. Indeed, in some buildings, such as prison
cells, it can be the norm.

Christian.