You'd also need to lock it in a way that disassembly of the bike will not bypass the lock.
Bye, bye Birdy! Unbelievably, this image was on Abus' web page (annotations are mine):
Locking a completely folded Brompton will require a looooong U-lock or a chain. I might suggest placing a U-lock at the red line and then folding down the seatpost & handlepost:
