Or even to switch to NATS
Within the UK military, Air Traffic Control Officers (ATCO) hold a 'Certificate of Competency' this is not the same as civilian ATCO Licence issued iaw the Air Navigation Order.
A holder of a Certificate would have to pass the appropriate parts of the ATCO Licence syllabus to gain the Licence, usually at personal expense, then Validate at the Air Traffic Service Unit where they are based.
I wish your friend well in her endeavours but she should remember that when in the Armed Forces one cannot 'pick and choose' - the 'exigencies of the Service' will always be paramount.