Items left in letter boxes but marked "registered" would always go by normal post.
I do not known about handling of registered mail in the UK, but the U.S. "Postal Laws and Regulations" volumes contradict this concept.
From the 1913 volume:
And from the 1948 volume:
It is clear, in the U.S., the post office expected all registered mail to be presented at a post office for mailing and otherwise erred on the side of caution when something was dropped in a street box.
Here is an item I have posted before: