In reply to Mr-Cowdrey: All you need is external grade ply then seal the edges with PVA glue or some other sealant to prevent water ingress into the end grain.
I know that I'm stating the obvious but the epoxies used to bond marine ply are designed for use in a sea enviroment. Unless you're planning on building your wall above the sea it seems rather excessive and wasteful to use marine ply. External ply is a much cheaper alternative and will last a long time if looked after properly.
After saying that, I think that the actual board will be the least of your worries in building your wall outdoors. Your t-nuts, screws and bolts will inevitably rust and you'll need treated timber to build your frame.
Given the weather in the UK if I was you I'd buy a new bike and a good waterproof jacket!