Not used it yet but considering it myself for our trailer. The snags I have been warned about are the fact it bends more easily than ply so probably need to up the thickness over ply.
I looked at the manufacturers web site. It seems that 6,9 and 12 mm is held in stock and sizes upto 25mm are possible but given the 12mm is around £65 pounds the overall cost of 25mm will probably be double the cost of phenolic ply. Possibly minimum order quantities will apply as well.
I have a pack of treated decking boards 32mm which might suffice.
Phenolic ply would be £75 + travel costs
Stokboard ?
Chequer plate would need to be maybe 6mm which would cost £200
If it's anything like Ecosheet, a polymerised recycled plastic sheet 8' x 4' and 18mm thick, it'll have very poor tensile strength, will warp in sunlight, and snap under any traumatic loading. I've used lots of the stuff for the last three years, and it's OK for building vivariums, and for pig boards. It's the one thing pigs won't eat, apparently! It's totally waterproof, but I wouldn't use it for a trailer bed. Just not strong enough.