I'm not so familiar with the situation in Canada, but I am guessing that it is the norm to build your own cabin or buy one that has been purpose-built for tourism by someone else.
I think that (in part, perhaps, because Canada is so thinly populated) there is likely to be more leeway about self-building your own holiday home in the Canadian countryside, whereas British planning laws make this really difficult. So, people can add to the existing housing stock rather than fighting with locals over a limited stock. I think that makes a massive difference.
The UK, on the other hand, is a fairly densely populated country, and there is concern about concreting over more of an island that is already somewhat nature depleted, which in part explains the slightly nutty planning laws.
An alternative, of course, would be to build taller buildings, and shift towards things like mid-rise apartments in popular areas. But the British have never been very keen on flats, and there is a feeling that areas like the Lake District and Cornish fishing villages are supposed to "look nice and old fashioned" with no tall buildings. If you can't build up OR out, that's going to be pretty limited. Unless people go Hobbit-style and start constructing underground dwellings (hmm, maybe that'll be the next thing...?)
There is also the British attachment to old buildings - a lot of people going to Cornwall probably insist on an old cottage for its olde world charm. For obvious reasons, there is only a limited stock of these. In Canada, for obvious reasons, there aren't many buildings hundreds of years old so I am guessing the issue does not come up - holiday homes will usually be newly-built, wooden structures.