Motorway junction numbers were invented about 1965-6 (anyone know the exact date?).
Don't know the exact date, you are round the right time, but one of those who put the plan together came to talk with us at university in the early 1970s, and described the process. I may have written about this here earlier.
One senior guy had the whole idea conceptually, the project was approved, the adaptations of signing were devised, and the detailed scheme of numbering everything was done for all motorways, present and proposed.
The numbering started right from Central London, and was done in miles. So the M4 would start at Chiswick at J7, Brentford J8, Heston Services (they numbered the services as well) J12, Heston J13, Heathrow J16, etc. This gave an indication of distance, allowed for new junctions, etc.
All prepared, put up to the Minister for approval. The Minister said the government should be encouraging metrication, and for a new scheme it should be metric, otherwise he might look silly to colleagues embracing the new world order. Comments about all signposts, vehicle odometers etc currently only being in miles were brushed aside.
Back to the desk, redid the whole scheme in kilometres. M4 starts at J10, Brentford J12, etc. Put up to the Minister again.
Minister (same one) said there appeared much public hostility to metrication, scheme might be ridiculed in the press, he would look silly to voters, etc. Do something different. Anything.
Back to the desk again. Someone said "do them sequentially then". And that's how it was done.