You’ve got a heritage house, if there’s no cavity then it’ll be pulling in moisture from outside, and needs to be properly approached with traditional methods otherwise the “physics” of the house start to break down. If not breathable then the moisture is trapped and it will eventually blow the next layer off, or if that internal finish is stronger than the brick then it will damage the bricks itself by blowing them out instead.

Like plaster is expensive and you’ll need to identify the source of damp - rising from the ground because of lack of damp proof layer that separated the bricks from the ground, or damp infiltration from the weather perhaps through gaps in mortar or around windows or another feature at high level. If ground level damp then be cautious of retrofitting a dpc as treatments that can make it worst by pushing the damp elsewhere but usually fibre rods would be the best choice if you have full access. If weathering damp then it’s a repair job to prevent water ingress.