Looks like the WestBoard stuff that Hobbycraft also sells which is actually cheaper at 4 x A1 sheets for £10. I've used it successfully for the FT planes but you usually end up tail heavy and need to put bigger batteries and more powerful powertrains in them.
You can save weight by removing the inner (heavy) paper side which comes off easily using a ordinary iron at cotton temperature to release the glue.
The FT foamboard is usually available from certain model shops like the Leeds Model Shop but they are out of stock currently, awaiting delivery! (for the last 2+ months)
I also use the Hobbycraft stuff, and can reiterate - tends to end up tail heavy. You can cut out triangles of the panels aft of the wing to create a sort of "lattice" type structure (sort of like a simpler balsa frame structure) and then cover it with tape etc., or you can do as Caroline suggests and just stick a bigger motor/battery in it and let rip. I ordered a couple of speed build kits to get a taste of the "real" stuff, and it is a LOT lighter...
I am not sure if you flyers over the big pond have this I only discovered it when the local $ tree was out of regular foam board. It is foam board but used for presentations it folds up the main piece is slightly larger with 2 pieces that fold over the middle piece. It is not the standard foam board it is thinner but stiffer. I built a Baby Ray indoor combat plane out of it have not flown it yet still waiting for motors, it might be an option. It is denser and takes a sharp knife to cut it but weight wise seems lighter but like I said stiffer.