is your garage part of the house? If so it would explain a cavity, if not I would not expect your garage to be built that way. If you have exterior brick and internal block it may just be 9” construction.
If there is a cavity that does not have insulation there may be a reason why not, by using the wrong material you may bridge the cavity allowing moisture into the inner skin. Also you mention a ceiling with insulation above and introducing a heater, again if your garage is part of the house be careful of a build up of moisture/vapour
Garage floors are not level and must drain towards the main door. This in case of a fuel leak and stops it pooling inside the garage.
Building regs are there for a reason, ignoring them can be costly.