Why do the Police have so few ranks? Because they don't have an officer's and other rank's career structure. Although fast track is available, all are expected to join as a Constable (graduates on a higher pay band) and do their time of up to 5 years minimum to progress into the rank of Inspector - they have played with direct entry Inspectors but I believe that ACPO aren't very keen.
So really it isn't comparable...
LJ