I see this question asked regularly through the years, so looking for some up to date ideas.
I've just purchased an 1890's 2 storey brick house in the UK. The rooms are quite large (3.6m height and most rooms 3-5m across) The brick walls are solid and have no cavity. I can't insulate from the outside (planning approval + aesthetic). Interior walls are plastered.some have lining paper/paint/some weird rough finish. I am happy to hack back to the brick internally. I have recently moved in and am beginning renovation work so open to any change. The Nest is reporting heavy heating usage already and it's only October....
I've looked at a combination of thin and effective insulating material that can be applied to the interior of the external walls. Some cursory research shows many new companies starting up in this space but I don't know what is currently effective. I.e. there is 10mm "spacetherm" board advertised but other companies suggest a minimum of 100mm for effective insulation.
Also open to alternative suggestions.