Real Bromptons flare the bottom of their seat posts , so they wont come out past the Bushing on top.


I Have an Abus Made steel o chain w lock, hanging from my handlebars .

and one of their Folding Link Locks in place of a water bottle , on the bosses Bike Friday put in the top tube..



FWIW, a Sailor moored in the Marina , was in the LBS, his Brompton was in Need of an Overhaul ..

Of note, he flipped the catch lever, reinstalled, behind the seatpost QR, to be above the bolt,

and used the space behind the seat tube to strap attach the pouch for the folding link lock there.