Many of the Castle Air pilots do get credited (one even has his own fan club!!) but some from other companies don't! I know two pilots, from two separate operators who worked on the Alan Titmush "Natural History of the British Isles"; one got a credit, the other didn't (and was miffed!).
The BBC used to have a policy that if somebody's work had a direct impact on the programme quality, then they got a credit. However, it appears now that you only get a credit if you're in favour with the producer.
Cheers
Whirls