More or less right. Hackney Carriages never have any other status so are, effectively, always working. The driver needs to wear the seatbelt whenever he is looking for a passenger, going to pick up a passenger or carrying a passenger. At all other times he must wear it. So where a vehicle is waiting at the back of a taxi rank and moving forward to fill up a space which has been created, an unbelted driver will commit an offence as none of the three exemptions will apply.