Courtney, as I understand it the military police don't have powers of arrest over civilians (even ex-military ones) unless they are under AFA jurisdiction overseas, and so they have to call for a proper copper to do the business. The Attorney General (presumably) having decided to prosecute under the AFA, Civpol can compel attendance at a properly constituted court, which under the new system the CMs are.
That said, the impending hefty prison sentence will be served in a mainstream jail rather than Colchester, which will hopefully be the least pleasant option given the sort of people to be found there!