I used marine ply from my local good quality builders merchant (Ridgeons in Cambridge, with branches all over East Anglia). The ply had the appropriate BS markings and it was fine on cutting and so on - no voids, and bits of it have lasted well when left exposed to the weather! It wasn't much more than WBP ply from the same dealer. The only down-side compared with a supplier like Robbins of Bruynzeel is that the facing plies are not decorative; they are a plain white wood with little grain pattern. They are obviously hardwood, though, but not mahogany or whatever.

I totally agree that while Robbins might supply really high quality material, their prices are totally unrealistic if the decorative properties of the material are unimportant. Their delivery charges are in silly money territory, too!

As I said, I bought from a builders merchant with a reputation to defend, and if I had encountered problems with what they supplied, I'd have expected them to be on my side. That seems to be the critical point when buying marine ply from non-marine suppliers - reputable firms will make sure they source their ply from good suppliers; less reputable firms get whatever is cheapest (which may not be what it claims to be).