Yes, the straw and plaster was the ceiling of the rooms, fitted below the joists.
Good point on the thatched roofs -- I've never had to work in one of those, so I haven't really had to think about it. I can't think of anything in BS7671 that refers to thatched roofs specifically; it would be covered by the general rules.
NM is the American designation meaning Non-Metallic; i.e. equivalent to your TPS or British "Twin & earth." Often known in the U.S. by the name Romex.
