In '71, I was sent on a course on Metrication, but Industry did not changeover for some time afterwards.
In the early 70s, Leyland introduced the 500 engine which was their first Metric engine. This was going to be fitted into chassis such as the Leopard which was nominally Unified, but incorporated Whit form parts such as the 0600 and 0680 engines and their mountings. So possibilities of finding all three standards on one vehicle!.
Perkins started to go Metric, dual dimensioning drawings, in the 80s, and the first all Metric engine (Possibly apart from the yield tightened cylinder head fixings which may well have still been 1/2 UNF ) were the 1000 Series followed by the 500 Series.
To confuse the issue, when supplies were running short, Massey Ferguson bought in 4.236 engines, built under licence from Perkins, in Jugoslavia, which had Metric fixings, despite the original design being to Unified fastener standards!
So Britain's changeover to Metric was by no means a sudden death affair, depending very much on the introduction of new products.
Howard..