I spent a few days in York last year and did the trip via train / & my nice new Brompton. Left it locked up in various places at various times of day and evening; but ALWAYS to one of the numerous Sheffield stands that proliferate around York. I use a good solid close link chain lock which weighs about 2kg. and I still have my Brompton - ! I think much depends on time and place and obviously York is a very cycle orientated city.
But I still agree with the above posts; if in ANY doubt, take your Brompton inside with you. Which is much of the point of the design in the first place; Bromptons are allowed in places which would refuse other less elegant folders.

