I am uncertain on their policy regarding folders.
Part of the reason I bought the Brompton is because I cannot be bothered lugging a huge lock around, yet they insist that an expensive lock is used at all times.
On train I just use an overpriced £12 impulse buy from
Evans, if at all, other than that I see no reason to lock it up when it is normally ridden, indoors, or safely by my side.