Building it here should be fine because of how small it is, would almost certainly be seen as a temporary structure and classed as similar to a garden shed. The problem is you cannot legally live in something like that.
Health code isn’t likely to be an issue in the UK as I don’t think we have anything overly strict that would matter. Environmental health exists but that is more of a concern if you are leaving a pile of waste that is attracting rats to the extent its causing a problem to other people. Been to events that use a shit pit before. Some you shit straight into the pit and others collect it into a septic tank and dump that into a pit later while marking a fence post near the pit to avoid digging in the same location next year. Does make me wonder how long it takes to rot down, 100-200 peoples shit from a whole weekend dumped into a pit and buried.
Then again maybe it isn’t allowed and we were just doing it anyway.
I have wondered about that kind of thing. But 13 plots of land is going to cost you a huge amount, at that point you can buy a house.