BDN has been in this game for years - the Jaguar GR1b springs to mind, circa 1995, and it's currently in the business of "upgrading" Chinook HC3s to a modified HC2 standard so that somebody can finally use them. Before BDN, Farnborough used to do this sort of thing also - the SK Autocat pods used in the Falklands in 1982 were Farnborough lash-ups.
It does seem dodgy, but it's also been going on for years - not least with all of their own research aircraft which have never been subject to much, if anything, in the way of external assessment.
How do they do it? To be honest, because they're pretty damned professional about it, and yes chinese walls. Does that always work?, good question. To be honest the attempts of BDN to bid left, right and centre for work that is outside their true competence in recent years (tanker fleet?, management of civil registered research aircraft when QQ has no civil approvals) has been unedifying at times and does a really very competent organisation no great favours. On the other hand, rapidly prototyping and modifying military aircraft is something they've got a lot of competence in, but it would probably be wise for them to be truly independently monitored.
But by whom?
G