Dry lining is a waste of time and money!
When adding insulation to an outside wall you should think what will give me the best result.
Internal of external.
If you take the internal route, you loose space but, you have a room that will heat up and cool down quickly.
If its on the outside it will keep the existing walls dry, making them warmer, it provides a heat store, meaning that the wall will warm up over say 72 hours but, will then hold its heat taking a long time to cool down,
If you are in and out often go for internal.
If you are retired and someone is there all the time go for external.
A 5 inch thick layer of polystyrene will save most of your heat but, first of all seal all the holes in the upstairs ceilings and outer walls, make sure all windows fit properly ditto doors - these latter will save you money today.