I am a regular user of Man at least once a week and live in West Yorkshire on a rail line with direct links to the airport. If I use the train I would have to either get taxi to nearest mainline station - there is no parking or none that is safe anyway and then get train. Other option again taxi to local halt again no parking then change trains on route. Bus to station you say - well we have 2 a day which come within in 2 miles of my house to coincide with schools. Car journey is 45min park in short haul, or when long haul airline sends driver and car, simples. Therefore the only time I would set out to deliberately take transport would be if there was some totally unforeseen event which for the life of me I can not think off.
However I also have a flat in Central London where I happily use public transport (there is so much of it !!)to and from airports, and on a day to day, as little point in having a full time car there, which may explain the high use of public transport to gain access to SE airports by others. If in London you already have the best public transport in the country even if the population of that city do not believe you.
Cheers
Mr Mac