The reporting of WC's police career in the media is all a bit confused. My understanding of it is that he had been a police officer for about eight years with the Civil Nuclear Constabulary (CNC), protecting Dungeness Nuclear Power Station in Kent. It is also reported that WC was a special constable with the Kent Constabulary.
The CNC is one of a number of non-Home Officer forces in the UK that have the responsibility for protecting high risk sites and high risk road movements to and from those sites. The CNC are routinely armed and therefore this is how WC will have become an Authorised Firearms Officer (AFO).
As the CNC is not a Home Office force, WC would have been unable to apply for direct transfer to Met Pol. Essentially CNC does not provide the all round policing experience required of a police constable, it is very niche. Therefore WC 'should' have started as a probationary police constable, attended training school and completing two years probation before being confirmed as a constable. I have to wonder in these times of being able to join the Met Pol directly as a detective constable, whether WC bypassed normal probationary training and was recruited directly into the DaPD?
I have read that WC joined Met Pol in 2018 and served at Bromley in Kent (Met Police area). I don't know how accurate this is. Although it would mean that he would have become established sometime in 2020, when he would have become eligible to apply for specialist police roles.
What is unclear is if WC was able to retain his AFO authorisation with the Met Pol during his probationary period. Once this would have been very unusual but it may have come at a time of police shortages of AFO's, together with the increased terrorist threat and an officer who was already a qualified AFO was snapped up.
I hope that provides some clarity on the process that I am aware of.