Actually, the above is not quite true, at least not in the "Kingdom".
Good quality WBP plywood can be of a quality far higher than Lloyds approved marine ply. In most cases, you just pay extra for the Lloyds stamp.
It is an urban myth, fiercely defended by some, that Marine Ply is better quality than WBP.
My boat is mostly made out of WBP since a lot of marine ply I found was pretty low quality. I put a higher quality wood in, but as a self build I didn't require the Lloyds stamp, so it was good all round.
If you want Top Quality AND the Lloyds approved stamp, then Robbins Timber in bristol is the go to place. But for much of my build, I used an Okoume WBP Ply from Totton Timber in Southampton that had a higher rating than any normal marine ply - the only step up would have been the Robbins stuff - the wood wouldn't have been better, it'd have just had the stamp.