Dont involve building control coz there are regs, once you have removed a certain percentage of plaster from the interior or even exterior you cannot put on like for like coz it probably wont have the same U values, so you will have to go with insulation boards. If its a terraced house i wouldnt recomend dabbing normal boards coz cold spots will show up, where everyone of your dabs are, go with sand and cement on the walls then skim.