There are enough people who would be spooked into coughing up on the spot (there must be a lot of punters not on here for the number of EE girls/barebackers etc to still be in business), and I reckon it would be easy enough once you'd scoped out the location to keep an eye on people arriving on the hour and leaving an hour or a half hour later and making an educated guess as to who to threaten. Still feeling skeptical about the OP's reporting of it but then I'm not losing out as she wasn't up for consideration in any case.
Happened to me at a Manchester Parlour (admittedly easier to spot punters there) a few years back with a guy threatening to follow me into work as I had a suit on (and it was mid afternoon), the punt was a pick me up after a car crash job interview so just got onto a train and didn't worry about it.
