On an old house like this, the gains really start showing once you combine airtightness work with decent insulation boards rather than just weatherstripping. I’ve been doing similar bits room by room and finding suitable materials locally was a pain. Online Insulation had the sizes and densities I needed for retrofit (rockwool slabs, PIR, insulated plasterboard) without having to deal with the DIY sheds’ limited stock. Worth a look if you’re going that route.