Houses built before the 1920s usually have solid walls, a single thick skin of brick or stone with no gap inside. These lose more heat than any cavity wall, which makes insulating them the largest fabric improvement an older home can make, though also the most involved.
Two ways to do it
You can insulate from the inside or the outside. Internal wall insulation fixes insulated boards or a stud layer to the inner face of each external wall, which is cheaper per room but eats a little floor space and means redecorating and moving sockets, skirting and radiators. External insulation wraps the outside of the house in an insulating layer and a new render or cladding finish, which performs better and leaves rooms untouched, but costs considerably more and changes the look of the building.
The saving on offer
Because solid walls leak so much heat, the potential reduction in heating demand is large, often the biggest single fabric saving available to a pre-war house. The flip side is the upfront cost, which is high enough that the payback runs over many years rather than a handful. It is best thought of as a long-term investment in comfort and value as much as a quick money-saver, and it pairs naturally with other work like a re-render or an extension.
Mind the breathability
Old solid walls were built to let moisture move through them and dry out. Wrap them in the wrong materials and you can trap damp inside the masonry, causing rot and spoiled plaster. With older and historic buildings especially, use breathable insulation systems designed for solid walls and take advice from someone who understands traditional construction, rather than the cheapest modern board on the shelf.
Start with the cheap stuff first
Given the cost, solid-wall insulation should usually come after you have done the loft, sealed the draughts and tackled the easy wins. Those give quick returns for little money, whereas walls are the big, expensive finale. Do them in that order and you are never spending thousands while pennies are still leaking out of an unsealed letterbox.