Thanks all. We ended up getting a few packs of the £7psm cladding from wickes and tried it out on one section. It looks rather good, but it's a bit too white for my liking so we will definitely stain it at some point. I've put a load of extra battons up behind the cladding where the kitchen units will go. And a big chunk of OSB where I'm expecting to mount the TV and another unit.
We are toying with the idea of using different materials in the front end of the horsebox. Someone suggested plasterboard....but I've not seen one conversion where plasterboard has been used, I'm guessing this is mostly because of the weight? I love the idea of a pallet wall, just need to find an easier way to prep it 1st.