The vertical sides of the slab shouldn't be on show like that. It should of been strip foundations with brickwork showing from ground level up to the top of the slab..
It would probably be strong enough, but a bit of a botch..
The easiest way would be to knock of the top course of internal brick, and line the cavity with visqueen, and fill with concrete level to the top of your floorboards. I wouldn't worry about the thermal issue...
A gap is left to make it easier to run cables/pipes behind, and so the flooring can sit on it, without the plaster covering the joist, but isn't essential. I'd just skew nail the joist & fit your hanger the best you can.. pack between the wall & joist when fitting the strutting.