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Boats are truly quite expensive to maintain, though if you have some mechanical skills, or are willing to learn them, it can be a lot cheaper. And obviously if you just moor up somewhere and pay the fees you never have to worry about the engine and that. Whether the maintenance of a boat is actually any more expensive than the upkeep of a a house you own? Probably not. I don't know a huge amount about boats, but I'm sure you could find a canal boat forum somewhere and learn what to look out for when buying one.
The cheapest way I could think of is buying a used motorhome, or like >>7380 says, a bus conversion. The advantage of a motorhome is that you can easily use proper camping sites, which can be fairly cheap on the off season, and if you're willing to go off the grid a bit (get a nice camper with a good leisure battery system or even a generator out the back) you might have a very, very, small overhead indeed.
Nobody's ever going to kick you out of a layby or anything like that, you'll find a lot of places where you can park up and nobody will bat an eye. Particularly in Scotland, where wild camping is considered a legal right, rather than trespassing as it is in England. You'd still likely have to move around, but that seems part of the charm to me.