Yes, I thought that wagon should have looked much newer too...lol
Both the Mk3 Cortina and Avenger wagon had "K" rego plates, which ran from Aug 1971 to July 1972.
The show was widely bagged for inaccuracies, the main one being the MK3 Cortina which was supposed to be a 1971/72 TC 2000 GXL, but was in fact a later model TD 2000 E with GXL badges!
I was surprised to see Austin Allegro's in a 1973 TV show as we didn't see them here until 1975. They were released in May 1973 in the UK with the quartic (squarish) steering wheel (as seen in the show) but soon replaced in 1974 with a normal one. The "L" rego one used in the show was spewing blue smoke, so had obviously had a hard life in its few months in the force...haha