This seems like a weird hangup. Is it likely?
Taking these films back in the 60s might’ve been illegal, but has anyone actually been prosecuted for it in modern times? Haven’t other lost episodes been recovered from ‘illegitimate’ sources without issue?
If it’s a real risk, it also seems weird to me that it’s apparently known that some people have these. Like, if there was really appetite for prosecuting them wouldn’t that be enough to start an investigation?
The BBC can be very pigheaded, i.e. offering no incentives for people coming forward.
Even without losses, they have a trackrecord of stockpiling a lot of old content but not making it available to the public. I doubt this would happen to Doctor Who but it would elsewhere. You would think with streaming that the BBC could make a lot of obscure old content available, but they don't.