I can't recall seeing one of those locking devices in the UK since self-service became the norm. As WON said, the attendants (I think that's what we called them) used to use them.

I reckon the device is a type of 'pawl' - "A hinged or pivoted device adapted to fit into a notch of a ratchet wheel to impart forward motion or prevent backward motion."

They are usually reliable, there's just a 'catch' to it .