Anyone know roughly how much a boat roughly big enough to live on and mooring would cost?

Mooring fees vary greatly as you say, and can be a crippling cost. Have a look at r/sailing, but that does seem more American-centric. It's something I casually look in to doing, pure folly, but if I was doing it I'd be buying a blue water sailing yacht and using it to travel the world in. I reckon £100k would buy a boat fit to do that but need some restoration work/alterations that I'd do myself. Be aware that maintenance can be a bastard though. Water wants to destroy everything and it's a constant fight against it. r/narrowboats is also worth a look and should give a real idea about running one of those. There are probably a pile of Facebook groups as well. There are for living in vehicles so I'd speculate there are for boats. Have an ask in some of those as well. Get as much info and opinions as you can. I'm going to be living in a truck and have spent over a year planning that. A boat adds a whole new level of complexity.