Dormer bungalow retro-insulation advice

Some advice please. Lucky enough to have a holiday home that's a dormer bungalow built around the early 2000's. In fairness to the builder its a masterclass of shoddy technique and sourcing of the cheapest material from the 80's. Some retro insulating has been done and the cavity walls have been pumped with bead insulation and the apex of the roof/attic insulated. However the dormer part upstairs is totally uninsulated. The crawl space is ventilated and as such the only separation between the rooms and crawl space is the plasterboard!. There is no insulation between the floors either so that's well ventilated to outside air temperature as well. So basically upstairs is either Baltic or like a sauna !. I understand that holes will have to be cut in the walls to retro insulate the dormer part but what type and method of insulation should I be looking at foam board /spray / rolls etc . Basically I'm looking at best bang for buck while trying to reduce the amount of destruction/rebuilding. A grant was claimed for the wall and attic insulation so pretty sure that's not an option ..TIA