I think that you will find that as a result of the Haddon-Cave inquiry into the Nimrod crash in AFG, the need to hold a professional status such as C Eng is becoming a key requirement for specific positions in the services.
There is currently a push within the RAF closely followed by the RN to get people registered as professional engineers.
It will soon be a case of 'if you want promotion you need to be registered'.
This requirement is now also spilling over into the UK Defence Industries.