It occurs to me that, in the Beiderbecke Trilogy, Trevor's van, a two-seat Bedford Beagle, regularly carries three occupants, with the extra one loose in the back, once whilst being tailed by a police car. This fact is never mentioned, even up to the final episode, filmed in 1988, when the van carries four adults and a baby in a carry-cot to the seaside. It's not done for any comedic effect, just as an everyday occurrence.
During that same period, I had a two-seat pickup with a fibreglass cover over the back. I would collect my parents from Lincoln. My mother had the front passenger seat and my father was on a car seat placed in the back (rear-facing), against the back of the cab and 'stuff' would then be loaded around him and he would be instructed to maintain a low profile. I did have some concerns about possibly meeting a jobsworth cop, but I never did.
The 'mindset transition point' may have been 1991, when rear seat belts became compulsory to wear - hard to do without there being a seat...