I suspect that it is - the Celotex must surely be over 50mm thick, and that timber looks like 89 x 38 CLS.
Interestingly I can't see any fixing for the bottom RH noggin that the back boxes are on, but the top one has just one screw/nail - presumably it's going to rely on the faceplates being clamped tightly to the plasterboard to stop the whole lot rocking back and forth.
If you look at the far end there are fixings going in diagonally through the noggin into the vertical. Maybe same arrangement on bottom RH?
Gonna be fun cutting the plasterboard for that lot.