Henry, one good thing at least ... you are well rid of that B&Q "pretend" flooring!
There are two ways of looking at this. There is generally nothing wrong with isolating the new (hardwood?) floor covering from the substrate. It's quite common to introduce a damp-proof barrier of some kind before a floor finish is applied. Hardcore or even back-filled rubble under the floor needs no damp-proofing in itself. The new floor is the only thing that needs this to protect it.
In which case, a good through-draught will soon remove most moisture from the substrate. Although, as I said above, it need not be bone-dry. In fact, because it is in contact with the earth below, it will forever contain moisture.
The other way of looking at this is to do as you suggest, and let the back-fill breathe through the floor. However, unless the filled void is subject to crossflow ventilation, breathing will be restricted considerably... rather defeating the object. The only way around this is to introduce e.g. airbricks between the fill and the outside world. Though, I can't tell from here if this is even practicable.
Once airbricks are in place, the new floor would have to be installed in a breathable way... to create air movement through the airbricks and up into the room. Obviously, the new floor covering could not be glued or bedded on bitumen for this to work.
If you did want to go for breathability, I would suggest fixing 25 x 50mm treated battens at 400mm centres to the subfloor (concrete?), then secret nailing the new hardwood floor to these. Not always possible of course, due to the extra height of the finished floor.