I lock mine on the train, sometimes I can’t sit close enough to keep an eye on it. A few months ago someone tried to pick it up as they were getting off the train.
Only carry a mini d for mine if I’m going to the football, and want to lock up outside the ground.
Do stand next to it on the tube/train as someone walking off with it whilst I’m sat somewhere else gives me the fear!
Why would you need a lock for a Brompton?