How to insulate this roof?

[apples12]

Hi all
We've inherited a partially built single skin concrete block outbuilding and I'm trying to figure out how best to insulate the roof/ceiling.

The current felt roof is going to be replaced with an epdm roof right over the top after being overboarded

I have attached photos of the current roof, I've looked at warm and cold roofes and the issue is the builder has closed off the roof above the wall for some reason so I'm assuming it will be very difficult to have a cold roof with air being ventilated via the soffits

My only other option in my mind is to fully fill the voids with foam type insulation (kingspan) and then fit a vapour barrier before plasterboarding?

So the sandwich would be

1. Outer roof skin
2. Foam insulation
3. Vapour barrier
4. Plasterboard

Can anyone advise if this will last or should I be thinking of other options?

Thank you

 

[noseall]

Fully fill between the joists (Celotex etc) then robustly foil tape all joints. Foam any large gaps first.

Fit 50mm layer (Celotex etc) across the joists - foil tape all joints.

Fit duplex boards.

The above still may not satisfy Building Control.