It's a funny picture, but Brits DO come in that size, so I bet Brompton seatposts can accommodate. Their stems certainly can.

...as opposed to some of these Asian folders. I've had 3 in a row now where for me (5'9, 31" inseam) to get full leg extension, the seatpost is maxed out in height. It's almost as if these Asian engineers can't imagine how long our legs are.