<<The answer is 121.5 >>
I would suggest that in the airspace in question that is very definitely NOT the thing to do. In a busy ATC environment, changing frequency without saying anything could be immensely dangerous.. but I'm sure that all professional aircrew know that.
If you change to 121.5 the RAF people will be faced with a major problem - someone in a busy CAS environment is suddenly working them unannounced so they have to go through the process of liaising what's happening with the controller of the sector where the aircraft is located so the workload is multiplied many times. I know that - I've been at the other end of the phone wondering WIHIH and desperately trying to avoid someone working the RAF.
Lastly, the controller who was working you is suddenly faced with a "radio failure" to add to his problems.
If you squawk the appropriate code it will be seen immediately..