Anyone know if in England, such a wall needs to be predominantly fire
resistant?

Stripping out the plasterboard now, to get more celotex on the underside
of the roof.

However, I'm interested in the idea of running 3' high 18mm plywood
around the entire room, routed with grooves to look like plank panelling.

Some sections would be removable and we'd make some secret doors for
access to the loft voids.

Before I go and bother the BCO, I'd like to check out some ideas so I
can go with a plan likely to meet approval.

It's one dormer room, about 2/3rds of the roof space, on top of a single
storey bungalow (so the dormer is the first floor) and only one room up
there, a sitting room.

So this ply would be the primary layer between room and attic.

Cheers for any thoughts...

Tim