This Bedford CA is based on the 15 cwt series which was produced between 1959 and 1969.
The Bedford CA had a 90 inch wheelbase, quite small by today's standards, so they launched a 102 inch version which was designated the CAL, hence the CA became known as the CAS.
The Post Office bought its first vehicle a Maudslay in 1907 and presumably the red on the vans was chosen because red had been used pillar boxes since the 1870's to increase visibility; originally they had been green.
Since then the The Royal Mail has run many thousand of vehicles delivering mail door to door across the country. The Bedford CA chassis found itself used as the basis for a succession of motor commercial vehicles so it is not surprising that the Royal Mail