That isn't strictly true. There are other ways of getting to an airport. Bus or coach, most major airports have decent bus services, often using vehicles modified to take passenger's baggage. Rail (not Glasgow); many airports, including LGW, MAN and BHX have excellent rail connections, especially BHX.
There are ways around the drop off charges at most airports. Top tip for BHX is to drop off at International Station, and your passengers can take the free overhead rail shuttle to the terminal. Dead easy! And there is a free short stay drop off car park at the station too.