It's nothing to do with experience, there is definitely a problem caused by the poor quality of many UK military VHF transmissions. I doubt whether it is the use of antiquated equipment since the problem did not exist when they used the old 'minicom' system. The start of the problems occurred when the 'minicom' system was replaced by more modern equipment.
Sometimes it can be bad and a definite flight safety problem. I remember a few years ago having to divert around a military zone because the transmissions were totally unintelligable, even with a female controller.
For a couple of years now I have made a point of telling the controller of the poor quality of their transmission and asking them to notify their supervisor. Maybe if we all did it every time we experience a problem, it might prompt them into sorting things out.
Submitting a safety report is a good idea and the CHIRP system is a very simple way of doing that. It can be done very simply online or by post.