What make up of cavity and insulation would allow me to build a cavity wall with an overall thickness of les than 300mm. Outer skin is brick and inner skin can be any type of block. Internal finish can be wet plaster or plasterboard/dab. The objective is to create a smaller cavity that 100mm but still meet the ).28 U value rating.
Solar blocks (100) 85 cavity with Dritherm 32 plus d/d should do you. Keep the blockwork really straight and the dabs small and you'll be close to your 300