I've googled RTI Forensics, for example. Securing a job there would probably pave the way for a post in the AAIB, although I suppose that the 'ethos' of such companies is quite different from the AAIB's one (I'd say that they are compensation-driven instead of knowledge- and safety-, and when the big money is involved, I start getting uncomfortable).
I doubt the comparison is quite as straightforward.
As someone already posted here some days ago, the AAIB has to liase with lawyers and insurers for the families of the dead, etc, and the final report comes out only when nobody objects to it. So there must be all kinds of commercial pressures. Which is not to say they modify any of the facts under pressure - no forensic service should do that - but a lot of speculative stuff is bound to get modified or deleted. And a lot of accident reports - especially in GA; no CVR or FDR - are largely speculative..... pick any 10 AAIB reports on all-fatal GA crashes and at least half of them will not reveal the definite cause. They just cannot tell.
The AAIB went through a phase, a few years ago, of picking up the use of foreign licenses and making less than complimentary comments on them. This was totally uncalled for. It seems to have stopped but either somebody in there had an axe to grind (plenty of those about among the ex airline aviation elite, who tend to have certain views on the business jet community etc) or they came under pressure from the authorities (the CAA or the DfT) to make a political point or two.