Its Celotex you are thinking about. Made by Celotex - UK PIR Thermal Insulation Manufacturers You want the vapour barrier on it too. Essentially anything that has a cold surface will suffer condensation. Regardless of how well ventilated it is. There will always be an element of interstitial condensation unless you provide a ventilated air gap between the insulation and steel. Failing this make sure the gaps between the insulation boards are air tight and sealed to prevent air coming into contact with the steel. The interstitial condensation occurs at the point of contact between the insulation and steel and as long as well sealed you should be okay. Furthermore ensure the internal temp does not drop below 15degC and you will be fine. 20mm will be fine. This should give you a U Value of circa 0.35W/m2.K which is better than the 5.8 without any insulation. Will help keep the shed cosy and your stuff nice. Put passive vents in rather than a fan.
Think you'd do well to heed the advice from the 3 brothers who each built a shed,but from different materials. If i remember correctly, the bickbuilt structure was far more secure than the other two. (Think they may have been HOG members.)
