If you do decide to remove the plaster then it would be an idea to use insulated plaster board since it is a solid wall, Which has vapour control layer built into it.
We have a whole front wall of 'all snapped headers' in my parents house.There are a lot of Victorian houses with cavity walls built in Mock Flemish with snapped headers. Headers are not always a sign of a solid wall.
The houses in my road were build with 9" solid walls, Flemish bond, in the mid 1950s.Houses were still being built with Flemish bond facework 9 inch solid up to the mid 30's.
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