I think that is plenty. These seem to be crimes of opportunity**. There are no stories of tooled up professionals roaming trains looking for Bromptons, and if there were, they would not find many. Also, it would be very risky to take wire cutters out and hope no one would notice. It would only take a slightly loud question from someone noticing if for the owner - who will only be a few meters away - to hear and respond.
I think this is true, but be under no illusion that tooled-up professionals are NOT roaming the trains.
A friend of mine had his Brompton stolen from his destination station. Obviously he normally takes it on the train with him, ut had a dental appointment that morning and locked it at the train station while he went to the dentists. When he came back, it was gone. CCTV footage showed a gang of people dressed 'as cyclists' getting off the train, going to the cycle racks, cutting free several bikes and then getting on another train with 'their' bikes back to London.
It seems like a lot of effort to me, but I guess outlying stations are a prime target as the bikes are left there all day.
So I guess they might take advantage of an opportunity to nick a bike from under your nose if you're not paying attention, but they're more likely to pick on those people locking them up for the day.