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Brick wall. Dead flat inside.
The idea of just putting some insulating plaster board over the brick wall and then skimming it is appealing. Especially if I can "lose" about 150 mm. And then get two warm rooms for the kids.Will think/discuss.
Obviously concerned about prospect of mould/damp between the new wall and old wall.
Chalfie
What is the outer wall made from, is it 'dead flat'? You can get insulation ready bonded to plasterboard which can be glued to a flat wall.
You're in a flat, so unlikely you can avoid thermal bridging/ cold spots (eg. If you line the wall, the floor/ceiling/other walls are still in direct contact with the cold external wall, depends how they are constructed though). So no point going super thick, in my opinion. Doing a single room is very different to, for example wrapping a house completely in external insulation. Even 50mm will make a massive difference to heat loss as well as comfort, plus board, you could lose less than 100mm from the room.
If you cannot ventilate a void (to the outside), don't have a void. Insulation as close as possible to the wall. Vapour seal between room and cold wall as much as possible.
If you don't like all this synthetic vapour barrier stuff you could go sheep's wool/wood wool and hemp render... but good luck finding a builder.