Hi Brian, I've used Totton Timber a few times.
Their Gaboon WBP Ply has been very good quality in the past, I used it for Caitlin.
Some of the "Marine Ply" is of dubious origin, some of it was described to me by one of the senior staff as "Cheap far Eastern Crap with a BS stamp"
They usually have a wide variety of , Marine, WBP, Gaboon, Finnish Birch, Latvian Birch etc. in stock
so its worth going and having a look.
It really depends what you are building and what method you are using. For Clinker Ply I'd go for the best WBP or Marine Gaboon I could afford, If its to be sheathed with epoxy/cloth then I don't think it really matters what is in the middle of the sandwich.
Cheers Graham