Wall insulation

The external walls, front and back, are of two thicknesses of solid brick, with no cavity or insulation of any kind. These walls feel cold to the touch in the winter, and must be responsible for a lot of heat loss.

The intention is to add several inches of insulation on each external wall, with plasterboard as a new top surface. I need to resrach thicknesses & materials, and also the ventilation requirements between the wall & the insulation; do I need an air gap?

In most cases this will require moving of radiators, although I intend to look into the possibility of incorporating underfloor heating when insulating the floor.

I'm hoping to be able to remount existing victorian window woodwork to line up with the new walls, although I'll have to experiment with the first one - pictures to follow.

The half landing will be a problem; although a large expanse of internal wall, the loss of several inches from the turn of the stairs could be quiet restricting. This may lead to a compromised insulation thickness, although keeping the hallway & landing at a cooler temperature will minimise losses.

I also want to lookinto how much the house heating dries out the bricks in bad weather; could wall insulation lead to a long term detoriation in the external walls themselves, if they don't have winter warmth?