Your downstairs rooms in the pics do not offer much in insulation opportunities, owing to you fabulous original features there. I would not touch the external or internal walls downstairs from an insulation POV as a result. The only improvement I would consider in this respect would be if the walls were in need of re-plastering, I would use a thermally efficient plaster (such as a lime plaster designed for this situation), but the gain would be small, so only worth it if you are re-plastering anyway.
Assuming your floors downstairs are floorboards, you should insulate under them. I am planning to do this in our 1900s house using thermofleece (made from sheep's wool). Also, get a carpet with a decent tog rating. This assumes you can get underneath and attach netting under the joists.
Windows: They might already be double glazed, so will probably not worth replacing. Although even if they are single glazed, you're unlikely to see the investment back in fuel savings until the units fail. You can get a gain from fitting acrylic panels inside the windows, fixing with magnetic tape or catches.
Also, do not underestimate the benefit of decent curtains. Those in the pics look thin, and badly fitted. Thick curtains with thermal lining, and not hanging over radiators would be a benefit you will feel.
Upstairs, I would consider lining a wall with internal insulation, and/or replace the render outside with a more thermally efficient one. A neighbour has done his house with kingspan mounted on metal frames inside. While it has provided excellent performance, it runs the risk of moisture build up.
It cannot be emphasised enough that you must consider the risk of moisture when installing these sort of things. Check where the due point will be, and learn as much as you can on the subject before embarking on any major works.
The roof is a no brainer on insulation: Kingspan, with the gaps filled with expanding foam, but again ensure you have sufficient gap between it and the roof.
Get a new boiler, and rads, but consider the following:
The position of the rads - do they need to the under the windows?
Further savings can be made with a weather compensator rather than a thermostat for the boiler control.
Consider additional sources of heat: Solar thermal, heat source pump (I've seen adverts for hybrid systems with heat source pumps and boilers, so worth investigating), wood burner (if you have a source of wood).
An additional benefit is in things like pictures, book cases full of books, and wall hangings. When these things are on external walls, they all offer small thermal improvements to your house. So don't be shy in putting up plenty of paintings, and collecting books, and even better - get a wall hanging.
