"Theft-resistant"
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!
Ouch. I also live in a city where bike thefts are … fuck, just part of life. Which is a horrible norm to have to accept.
You’re right that a grinder-proof lock does not exist. Having read reviews and listening to bike shop owner experiences (they hear everything) … I have Kryptonites, motorcycle chains, Abus folding locks, but my personal favourite that I’m “trusting” (ha, delusions) is the Hiplok DX1000. Check it out. As good or better grinder results than nearly all others, it seems. But nothing is grind/pry/jack-proof.
Someone else in these comments: angle-grinder-resistant locks: https://news.ycombinator.com/item?id=47909705