> While it's not impossible to insulate such walls, it's generally inadvisable for many reasons

Lime plaster with cork in it is fine way to insulate these buildings.

It won't be as good as modern materials but is an improvement and is breathable

> Generally, the poorest and oldest people in society occupy these houses,

I own one of these houses – single skin, 200yr old farm workers cottage – I'm neither poor or that old

Much of the expensive old housing in London, Bath etc is single skin too