I spent a lot of my youth messing about in old bomb shelters. Most were, as you say, earth-banked - this provided added loose mass to 'soften' the blast, and a bit of 'shape' to allow it to pass, having less effect. I was never happy that few of these place had a secondary access - so, if the door became blocked, the occupants would be left in the hands of those outside, who may have other priorities, by that stage.
I have seen it suggested, with an air of confidence, that more people may have died in the UK from TB contracted in shelters than may have been saved from death due to the bombing - who knows?
If you ask people on the Big Island where in the UK, outside London, had the greatest WW2 bombing casualties, by far the most likely answer is Coventry, when it was Belfast, by a long way.