My suggestion is that missionchief adds a new mission and/or new unit, the new unit would be a Prisoner Transport Van as normal cars do not transport prisoners in the UK usually.
The new mission would be a transport, or you would just get a transport request and have to send a PTV to it.
Only the PTV unit would be able to transport prisoners and would have to have a minimum of 1 crew member and a max of 2 crew members.