Did your seatpost ever turn up? I ordered the same one (I think) and it was apparently sent back to the seller by the airline due to its excessive length!
It did, surprisingly quickly as well - ordered on Feb 23rd and delivered on March 7th
I've been meaning to loop back to this thread and report on it - but then there's not much to say - it looks just like the Brompton one, fits fine, doesn't slip and I've not noticed any issues whilst riding it
Thanks all for the seatpost comments - I've ordered a ti one from aliexpress - will see how it goes