Although external insulation is the best way to go you do have other options internally. You can use hemp or sheep fleece between the studs as well as rockwool. Do not use EPS as it is closed cell in construction and will not let the wall breathe.

You may also want to consider Sempatap lining paper and to decorate consider using Thermilate paint which has just got a BER approval.[www.exteriorinsulation.co.uk]