I finally found a company who supply the foam in the UK (Yippeeeee) and after a call to them they were so interested that i was put through to a senior technical sales person. They sell a lot to caravan companies as made up panels. These have alluminium on the outside and a finish ply on the inside.
The foam he suggested is what is commonly used under floor boards in house building which already has a perfectly cut rebate as in your picture (blue one) all the way round. Its available in 2500mm x 600mm (8' x 2')
When i asked about it only being 600mm high and would mean having more joints he was clear that a well glued joint is stronger than the plain foam and vertical joins can be staggered. They use a 2 part glue and in testing the foam always breaks way before the joint. The rebates fit together to make a perfect flat surface or a perfect right angle join. He explained that normally (under a floor) these are not glued together and a butt joint could slip where a rebate one does not.
He suggests that glueing two 1" pieces together is not a good thing and said as all glued foam joints "MUST" be clamped so the joint is under pressure until dry, on such a large joint a DIY builder could have a problem.
I said i was sure it could be done and he agreed but it would need a perfectly flat floor and perfectly flat weight so the same weight was distributed all over the join. He thinks with DIY when you lift off the weight the double foam would be wavy when viewed along its length.
I think he then realised i only wanted a few sheets so made an excuse to go as he had to quote on panels for 140 prison vans.
Two things he would not comment on was the use of PVA (TB2) and cloth covering and using GG on joints as he had no knowledge on this.
Company details:
I first contacted
http://www.styrofoam-online.co.uk and they are also
http://www.panelsystems.co.uk
lots of info on there