Insulation board on external walls.
Are you OK with the fact that it's very difficult to have cables or pipes behind and then coming through the insulation board, very difficult to have accessory boxes sunk into them etc?
The walls will be cold - even colder than they get now, and the interior of the rooms will be warm. When warm air gets onto the cold walls through holes in the insulation made for back boxes, cables etc it will cause condensation, which will lead to damp, possibly mould with spores which are bad for your health etc.
It's very difficult to get internal wall insulation right, but very easy to get it wrong and to start building up a load of problems.