The Preston Bypass (now part of the M6) was the UK’s first motorway. Subsequently the M1 was the first full length motorway, opened to the public by the then Prime Minister Harold Macmillan on 5 December 1958.

Margaret Calvert, 2010, BBC, The Genius of Design – Better Living Through Chemistry

We were invited to the opening of the M1, and we had obviously special permission by the police to go on this motorway. It wasn’t open to the public at that point. It was a beautiful sunny day, blue skies, and then just us, no-one else on the motorway. Then we’d see our signs up, and we thought, “Wow!”. They just looked so good. They were big, and they sat there and they had their own presence. We just went on and on and on and on. We missed the turn-off or where the actual opening was, and I think we just ended at the end in a ploughed field. But we had a great day. And that’s probably a big moment in my life.

Echoed in a November 2020 interview in The Spectator Laura Gascoigne writes:

…she does let me in on an amusing story about missing the turn-off for the official launch of the opening stretch of the M1. Kinneir had taken up driving halfway through the project — ‘It’s best to approach things as an amateur,’ thinks Calvert — and was the owner of a new Fiat 600. ‘We’d been told where to stop, a little siding, but it was a lovely sunny day and we were thinking how surreal the vast blue signs looked against the brown banks and we forgot. We came to the end and had to drive into a ploughed field.’

Above is The Secret Life of the Motorway (BBC, 2007). From 10:37 Margaret Calvert and Alice Rawsthorn each discuss the sign design.

The above video with plenty of examples of the type system was found through this article in The Telegraph by Andrew Roberts.