I have two Bromptons (a 3 and a 6 speed). Unfortunately, I've also had TWO stolen .. painful. This was years ago now and both were locked up in central London. The second time, locked via a "Kryptonite New York Fahgettaboudit", a good U-lock. An angle-grinder gets through these easily and they're battery powered. I don't believe ANY bike lock is safe now and never lock the Brompton up outside. Great bikes!

Edit: the top comment (at this time) has a youtube link that shows it fold/unfold in like 8 secs. I can see the genius in it now