Actually, Robert, at the time my Uncle and Aunt were caravanners, there was no speed limit in the UK other than a 30 limit in towns. Once you got outside the built-up areas, there were circular white signs with a diagonal black bar that indicated "No speed limit".
When I was out testing Norton motorcycles in the mid 1960s, there was a 70 limit on the motorways, but it was widely ignored. I don't remember any of the graduated limits as you left towns. Most of my testing was done on the smaller roads anyway, as motorways are boring on a motorcycle!