The barn

I want to improve the insulation of my old barn. I have underfloor heating but the external walls let out all the heat. It has featherboard cladding on the outside which has shrunk so there are gaps on the sides in lots of places. Behind the featherboard is about 50cm of celotex and some black membrane in places then plasterboard. From the inside, you can also feel the cold air in the holes in the wooden beams. What is the best way to improve it? My builders says I should remove the existing featherboard, batten and then put some sort of insulation material and then put new featherboard back on. The council will only let me use wood as I am in a conservation area. Any suggestions for the best type of insulation? I would prefer not to do any work on the inside if possible but that may be the best way to do it.