Brompton owners tell me this a lot, yet the only place I've yet been unable to take my 20" wheel folder was on a light aircraft (Soceta TB20 Trinidad, as it happens) and was because of the weight (Im a big unit myself and had taken up most of the available capacity on my own.) Seeing as its lighter than the typical Brompton they too would have been grounded.
I left it with the nice chaps at Lydd flying club who offered to stash it for me.
Trains, sleeper train, a coach, the underground, lifts, stairs, escalators, cars, vans, the bus (big london double deckers and smaller local swb buses), all of them crowded and many standing room only, its been everywhere.
And then when unfolded it gives me a modern disc braked, 2 piece cranked, 9 speed ride that the G Spot could only dream of.
The idea that something with a Brompton badge is the only bike that can do all this is a hilarious fiction that only Brompton owners seem to believe. Us riders of alternative folders just get on with it.