BS1088 is a far more rigorous standard than exterior grade ply.  Part of that standard is the bend or flex of the wood.  This is integral to the design.   Let me ask you this, what is the most expensive piece of your build?  And how long will you have this boat?  The most expensive piece is your time and I am going to guess that you plan to have the boat for many years.  Spend the extra on good materials, this you will not regret.  There is a really good sign that I drive by often, “the sweet of a low price is long forgotten with sourness poor quality” or something like that.

As for the wife, I appeased mine by building her her own “yacht” and now we are paddling together, at least once a month since launch!

Couple of the major differences is that the glue is water resistant, the flex I mentioned above and there are no voids.  Most ply has voids in the inner layers that are hidden below the outer vanier. Right after I posted I realized that I did not answer the primary question.

Hey Tony.   I agree that using the best woods possible would be the way to go.  Having said that, I built a small 8' rowboat in 1975 uaing ACX plywood.  No epoxy, no preservatives, just fiberglass on the seams using "junk" polyester resins.  That eas 30 years ago and I just rebuilt the boat!!!!.

So if you can afford the better woods and can dodge having your wife leaving you, use the good stuff AND epoxy based resins...   ~TUGBRUCE~

I agree with the above except the statement that glue in exterior is not waterproof.  I think that BS1088 and exterior plywoods use the same type of glue.  I think I read it in one of the books from my CLC kit but don't quote me, check it out before accepting the above.

thanks to you all, yes i think marine ply is the way to go for me more expensive but its gonna last, and give me peace of mind.

can you tell me what to look for when im choosing marine ply im lead to believe that having the BS1088 stamp isnt enough have been told there`s a lot of cheap stuff out there that should be avoided, how can i know what im buying what should i check for visually??

all your comments are really appreciated 

Exterior ply can be perfectly fine for boatbuilding. You may have some problems locating ext. grade that's thin enough though. The ply needs to be able to bend freely  enough to take the boat shape. I have no idea what's available in the UK but in the States AFAIK the thinnest readily available ext. is 1/4" -6MM which is altogether too stiff (and heavy) for your intended boat.

 See if you can find some furniture grade interior ply of the right dimensions. Furniture grade generally uses waterproof glue and has more plies than construction grade. I wouldn't hesitate to use furniture grade for bulkheads, coamings, etc.

You're right about the BS1088 stamp, means nothing these days. When you do find something that might work obtain a small sample and give it a boil test. Submit the sample to several iterations of boiling/freezing and if it holds up, Bob's yer uncle.

thanks for the info Dusty Yevsky