"Both the Army and the Navy have Cranfield trained AI teams, being trained on the same course as the AAIB."
The 6 week Cranfield course is the basic starting block. AAIB Inspectors then get continuos development and training on accidents (10 - 15 accidents per inspector per year). It takes time, and practical experience, to properly train an accident investigator.
The current military system with the Navy and the AAC, has guys doing 1 maybe 2 tours, then moving on.
If a MAAIB was to be the chosen way forward, then it would need to consider Investigators continuity