Indeed that restriction was still in force a few years later in my time but for different reasons as the security side of exceeding Mach 1 was long gone as an issue following Farnborough 1952.
Indeed! From the Flying Programme of a typical small RAF flying station "At Home" 18 Sep 1954:
It is hoped that the Flying Display will include the following:-
Breaking the sound barrier by a Sabre of Fighter Command.