There are two ways to insulate a solid wall internally – with rigid insulation boards, or a stud wall.
Stud wall insulation is thicker than rigid insulation boards, so it will reduce the size of your room more. But a stud wall is strong enough to hold heavy fittings such as kitchen units, radiators or wash basins. Insulation boards need fixings that go through them and into the wall behind.
Is the wall’s surface even? If the plaster is uneven or plaster has been removed and the brickwork is uneven, the wall must be levelled with a layer of plaster or render before boards can be fitted, so a stud wall might be a better option.