Ah, those holes are actually where I've dug up previous post-mounts from the structure you can see in the first couple of pics.
It'll be built up in sections. The floor is steel, wood, rockwool, steel again, a layer of acoustic plaster-board, then the normal flooring. The whole inside will have a vinyl wrap and then another layer of acoustic plasterboard green-glued on. The windows and doors are laminated acoustic glass. I have no illusions about this though, it'll be better than a 'normal' portakabin but it's not going to give me the levels of isolation that a brick-on-concrete building would.
But I'm not planning on playing real drums in there...