As an aside, since the topic has been raised, here is a list of carry-on luggage sizes accepted by the major airlines. The Brompton, largest folded dimension 58.5cm, exceeds nearly all of them now so in most cases would not be accepted as hand luggage...
https://upgradedpoints.com/travel/airlines/carry-on-luggage-size-chart/
It's seems as older aircraft leave service, the 747, 757, etc, the overhead lockers and cabin storage opportunities have, like chocolate bars, shrunk, so into a suitcase the Brompton goes.
Tape measure shows my folder would also fit in either of my larger suitcases, so could easily travel by air as passenger hold luggage rather than general cargo like a full size bike would. The cheapest suitcase I can find only which would take it is £59, so if I were on a tour I could just discard the case as I leave the airport, but we're well into the realm of 20p scenarios there.
This idea that a Brompton is the only bike that fits on trains, the underground, taxis, coaches, private cars, Submarines, Ernst Stravro Blofeld's escape capsule or is the only one suitable for air travel is utter bunk. Don't fall for it.
In fact, next time I'm down in Kent I'll take mine across the channel in my Dad's TB20 Trinidad just to prove the point.