The video the OP gave shows what I call a houseboat, rather than a boat, but there looks to be a Haswing electric outboard fitted, so they are movable to a limited extent. I am guessing that the RCD and UK version apply, but if his brother in law is building these in Poland, then these rules apply anyway. I've embedded his YouTube link below. They look rather nice. Won't fit on many UK canals, but if you can get past the planning system, then a lot of lakes, rivers and larger canals that would suit. If the size of the market is worth it for them with the Brexit hassles I don't know. Best of luck with the venture.