Matt, follow the link that you posted and see 'how to secure loads safely' it does refer to ratchet straps, so how would this deemed as being illegal?
Its just my opinion, but if a load is secured by a mechanism that is inadequate then it isnt secure.
If it isnt secure its unsafe.
I just go by what I feel and believe is the right thing to do. For the sake of a few quid Id rather travel safe than travel with a heightened risk of having 3/4 tonne of water coming through the bulkhead rendering the vehicle uncontrollable.
Don't forget the dynamics of a load of water compared to the dynamics of a similiar weighted load of cement is going to be entirely different.
If you want to travel with a DIY system feel free, just dont crash into me.