Whist your latter point is obviously true, I think one of the main reasons why DNOs (and suppliers) now favour external meter cabinets is that it avoids the difficulties associated with their personnel, or meter readers, gaining access to internal cutouts and meters.I wonder what the objection would be (by the DNO) if householders could pay for a more secure box. After all, this would be no more of an inconvenience than gaining access to a meter inside someone's house.
In answer to your question, I would expect that the DNO's position/response might well be that when there is an external cabinet, they want the contents to be accessible to any of their personnel, or any meter reader, at any time - which would probably effectively preclude a significantly "more secure" box (if every DNO employee and meter reader had access, it would probably not be, or remain, significantly more secure than the currents ones!).
Kind Regards, John