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They absolutely improve the thermal property but you have to look at the room as a whole as having thermal bridging issues need to be resolved. Have you got an air tightness strategy?
I have a home very similar era and solid wall and will also be looking at diathonite for internal insulation too. Also ensuring good air flow and ventilation even with natural insulation is still critical.
Limecrete flooring is definitely worth looking while also still looking at hemp for further insulation under that too. You could be looking at upwards of £100-£150 per square meter just for the limecrete, depending on the thickness but also factoring in the right floor system with that is key and what floor level changes.