It had a 50mm cavity but no insulation, the damp course was slate so no good, all the bricks in the internal walls where damp so replaced by blocks and proper damp proofing. The front however didn’t have any cavity so we put it back with one.
in the 50mm cavity walls we packed with insulation and internally used insulated plaster board to give the same properties as a 100mm cavity.
the insulated plasterboard might be good for your walls if you have any more decorating to do.
im with you on the gas, our old house didn’t have gas.