No disrespect to anyone doing these courses, or anyone running them if it comes to that, but have you ever looked at the standards laid down when jobs for air accident inspectors are advertised?
A basic requirement in the UK is many years experience in the industry at a very high level of flying and engineering, backed up with a knowledge of aviation which most of us mere mortals can only aspire to. It is no suprise that the expertise of the UK AIB is often called upon to assist with investigations which would normally be well outside their operational and geographical area.
In over 20 years in the industry, I cannot ever recall seeing that the attendance on an Air Accident Investigators course is a requirement, or even an advantage in making the grade in this particular profession.
I would suggest that if you are just interested in the subject, attend a more broad based airport operational course, where the subject is covered adequately - there are at least 2 major organisations that I know of who offer these courses in the UK.
But if you are serious about becoming an investigator, get in touch with the AIB themselves to find out what their requirements of their staff really are.