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I'm 6'4 and have a Brompton (s type!) for my commute - I wouldn't want anything else on the train tbh, as nothing else (I've seen) folds up as neatly on the luggage rack. Although if you have the telescopic post it will protrude onto the luggage shelf above, meaning you cant push all the way in (but this has never caused any real problem)
As for specifying the post on purchase, I brought mine through Condor and they let me tweak the spec for a longer post and a bigger chainring as part of the purchase (although I later realised the extended post wasn't long enough, so then brought a telescopic separately)
I'm 195cm and had a H6L with the telescopic seatpost. It did fit me fine, just looked unusual...
I sold it and wouldn't buy another in London though – impossible to lock up without theft so had to carry it everywhere which was annoying – I'd get the Decathlon £200 folder to remove the the fear.