Here's an A.R.P. Illustrated Practical Guide booklet, I've scanned a few pages you may find of interest.
The chapter on Domestic Surface Shelters I can relate to personally.
When we moved to Skegness we bought a Semi-Detached house build in the late 1930s, this was in 2002 which still had
one of these shelters still standing in the rear garden build of brick and had a concrete roof of at least 5 inches thick, it was
a type F as depicted in this booklet and stood about 8 feet away from the house. Next door had demolished their half of it
and as far as I know was the only one left standing on the street, there was not much room in side them you could get two
people sitting down or four people standing. they were about the size of a small tool shed and that's what I used it for.
How practical they would have been during the war I don't know, but if the house had been hit with a 500lb bomb I think
the blast would have took the shelter with it. As far as I know it's still there.it was in 2010 when we moved up North.