On the older houses I have worked on overtime, some have timber frame for the laths to fix to but unfortunately are not insulated, leaving a 100 150mm void, between the tmber frame and inter plasterboard. Others have the itchy 50mm insulation in the timber frame.
I wouldn’t recommend unglued polystyrene beads because not only will they slump over time as they settle, you run the risk of creatng cold spots if the beads don’t settle evenly or clump in some spots when you are tipping them in.
By far the best way is to tacle the job from outside by removing the tiles/felt and filling the void (leaving a 2″ airgap on the inner part) with sheets of PIR insulation and replacing the tiles with composite shiplap cladding. You also have the option of adding battens on the existing timber frame to accomondate thicker insulation sheet’s 🙂