empty pockets
I recently wrote to the AAIB with the same question - their response contained this description of the positions and qualifications required:
Inspectors (Operations):
These posts are recruited from professional pilots who must have a current ATPL and have extensive experience in more than one sector of aviation, but usually including large public transport jet aircraft. A test flying or management pilot background is an advantage. Rarely recruited under the age of 40 years
Inspectors (Engineering)
The engineers come from a variety of engineering backgrounds with a wide knowledge of aeronautical engineering and a suitable degree. They must be capable of investigating accidents across the whole range of aircraft, from microlights and balloons to public transport aircraft. The youngest age at which we recruit engineers is usually 30.
Other Posts
Mainly clerical dealing with administration of the Branch.
I hope this helps!
PS.