This Wikipedia entry on the Home Guard is quite extensive with many links:
https://en.wikipedia.org/wiki/Home_G...United_Kingdom)
At the very end of the Wikipedia article is a link to an
on-line copy of the Home Guard Manual by John Langdon-Davies:
https://archive.org/details/JohnLang...p?view=theater
You can change the way it is viewed using the controls on the bottom. (It was scanned two book pages to a single display page.)
The book went through at least 6 editions (this is a copy of the sixth edition and was published in October 1942.
On book page 41 (scanned page 21) is laid out the Powers of the Home Guard. They had the power of arrest, for example.
Starting on book page 82 (scanned page 42) are the procedures for roadblocks.
Beginning in Section “8 Challenging” on book page 83 (scanned page 42) are some detailed instructions on the process.
At night, the home guards were to use a moving red light as the challenge signal. If the person (or car) did not stop they would be fired upon. (I wonder if the moving red light was the initial procedure from June 1940.)