Just to balance things up a bit, don't forget that Bristol Airport will be, by a country mile, the largest generator of economic activity in North Somerset, and one of the largest in the WECA area. That economic activity employs thousands, if not tens of thousands of people, including pilots, cabin crew, ground crew, maintenance, ancillary, baggage handling, security, retail, etc etc etc. That generates millions for the national coffers in tax and national insurance, I've no doubt the airport's business rates is not a small figure, and of course some of those earnings will end up spent in local retail and hospitality outlets too.
So when you say the local authority (which incidentally is my local authority, so I do have a close and vested interest in the subject) shouldn't spend a penny on facilitating access to the airport, I'd much rather it spent it on that than bus lanes that buses don't use (yes really) and a sea front improvement scheme which everyone hates and is now scheduled to be reversed in part at further cost.
And don't forget that those 12m are using the airport because they want to go somewhere.
P.S. I have no connection with the airport, other than as an occasional user.