I think their planning is a lot more zoned than over here. I assume providing you build within certain parameters planning is automatically given. No silly planners going on about vernacular on a modern estate.
You look at the quality of design of houses here before planning was brought in (1940s?) vs after. Which houses look better?
Do you mean structurally or thermally? Structurally if they've lasted this long then should be fine. Must have quite a few intermediate walls? Thermally as @ProDave says be a hell of a lot better than nowt. Some modern houses still have 4" in the cavity.
I had tanking peel off 1 wall before tiling once. It was the external wall and I wonder if the plaster hadn't dried enough to accept tanking. I stanley knifed up 1 corner and peeled it off in sheets. The 2 other stud walls were fine. It must need a good dry surface to adhere properly. Ive used it on other projects no problem but usually on stud walls
Is it a truss or a rafter? Not to be pedantic but one is a lot easier repaired or replaced that the other. As it is probably be fine. How far from that horizontal ceiling to a purlin if that is a rafter?
Ive seen big commercial renovations where they just use metal stud with rockwool behind and in-between. This is on 9" solid brickwork. Surely if good enough for that be fine for modern cavity?