Insulating between the roof joists is known as "hot roof". seen it on a few builds I have been on in the past and yes, an air gap was left.
Why not consider getting cavity wall insulation done? definatly makes a differance and with the hot roof, you would not need to take up the flooring to reinsulate that. But if you do, use rock wool instead of fibreglass as it is three times as good like for like thickness
Cavities were done about 15 years ago. I'm not convinced they are a good idea in our area though. Lots of reports of problems caused by it bridging the cavity causing damp problems. Saw a map once of areas that were advised not to have it, and we were in it. Something to do with westerly aspects that see the brunt of the prevailing weather.
Depends on the type of fill and the area, if your walls are exposed to regular wind driven rain that can soak the outer brickcourses, then it can be a problem. In the real world this is not many houses