when you say lining it out with Kingspan seconds, have you actually thought about using Kinspan or Tata panels? There was a place in mid wales that used to sell seconds. All you would need is buy some 80/100mm steel faced panels get some decent self tapping screws, you would need to watch the junctions and the flashings, but that would apply if you use sheets. Oh and the other thing to watch, condensation and wooden purlins, you can get sheet failure, more an issue for cattle barns though.
If your serious i could try and see if i could ask some former colleagues, i used to work at Tata/Corus, if they still offer seconds, you won’t be meeting prime quantities and minimum order quantities by a huge margin, unless you’re not really building a garden shed.