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NotThamesWater
Yep that is correct. And I left the battens a bit gappy so nothing is fully fully enclosed - just in case. Kingspan/celotex/ecotherm all have guides on the subject. CharlieDIYte on YT also has a video - although he did dot and dab.
Your north wall/lowest point on the roof/air brick scenario is exactly the same as ours. Knocking back to brick wasn't an easy decision but am glad I did it - the room was small enough as it is.
I used these boards https://www.selcobw.com/products/plaster-drylining/plasterboard-accessories/plasterboard/gtec-thermal-pir-tapered-edge-2400-x-1200-x-37-5mm
Many thicknesses to choose from, I elected for 37mm which has 25mm of PIR insulation. It would not meet current U value requirements, but provided you aren't doing more than 50% of the walls in the room, you don't need BC sign off, and I felt the improvement with 25mm would be sufficient.