I suggest that you have a chat with your local authority environmental health department. They should be able to advise on appropriate systems and any planning requirements. You never know, there may be some grant available. There are loads of differing external insulation systems out there with some being mechanically fixed using proprietary fixings whilst others, where render is being used, are carried in a stainless steel cage, itself being mechanically fixed. If at all possible the half brick walls should be upgraded, killing two birds with one stone.