"Yes, and in all his life he only saw one sign."
Does he say that - must have missed that one.

You missed this.
"You couldn't get away with putting up signs like that now.
That's what I thought." ....until he saw that sign.
That makes it clear and obvious he had not seen one before.

You have provided conclusive proof that such signs are not "common throughout Britain" as you stated.
Thanks again.