Have a look at these two books: JP airlinefleets and bizjets
BUCHair (UK) Ltd
Usually used by enthusiasts for looking up reg's or whatever, but it's well researched and not expensive.
Plus, it doesn't only give you all UK airlines or bizjet operators, but WORLDWIDE with all contact details as far as I can remember.
I think air taxi ops and the likes are not listed, but it's a great way to start.