For a 16-seater, you'll definitely need a D1 license if you passed your driving test after January 1997. If you passed before that, you might have grandfather rights to drive minibuses up to 17 seats.

I used Glasgow Airport Transfers last year for a similar group size. They were very reliable and arrived 15 minutes early. For a 14-seater, we paid around £120 one-way from central Glasgow to the airport.