Using the very best cavity insulation, e.g. Kingspan K106 boards, you could get a U Value just over 0.18 with 12.5mm Plasterboard/25mm service void created by timber battens/100mm medium density blocks/90mm of K106/10 residual cavity/103mm brick. Total wall thickness would be 340mm. Adding another 25mm of K106 gets you to a U value of just over 0.15 which is Passivhaus standard.
