Well I tried this over the weekend. I am 6ft and a 31.5 inside leg.
The medium felt exactly like a C line Brompton, slightly cramped and everything a bit more tucked in. If I was to get this I would definitely rotate the bars up to get some more height and reach, even fit a riser bar.
The large felt, well large, but I could move around a lot more and with the Brompton bars angled back its just slightly, too big. I would want the bars about 5mm more back to be perfect.
However, if you ride it like a normal bike, as opposed to sit back and enjoy the scenery, the large allows a bit more weight over the front.
Neither size is really right for me, but the large requires less tweaking and doesn't feel small, but like a proper bike.
I rode the cobbles in Covent Garden and could not feel them, its just a blast to ride.