There's plywood, and there's marine plywood and there's Marine Plywood.
The local builders providers sells marine ply. Excellent. Less than half the price of the stuff sold by Waller Wickham. What's not to like.
Well - specifically, there's only 3 plys on the 6mm boards. And the veneer ply is as thin as paint, leaving a big fat ply in the middle with all it's strength along one axis. Not likely to hold up to much of anything.
So I bit the bullet and wandered down to Waller Wickham and got a sheet of 5 ply 6mm to make up the gussets and the centerboard case. I believe the ply they stock is Robbins Elite.
Who was it that said "Light, strong, cheap, pick any two". This stuff is light, strong, well made, but not cheap.
I didn't feel as though I even had a decision to make though, the builders yard marine ply was totally inadequate, even though it was stamped with BS1088.