From the latest posts of
@Gareth J and
@KenGordon I see that the situation in UK is very similar to Italy, and, I suppose, to France, Spain, Greece, Croatia, and possibly the rest of Europe.
Here these light-deco dives, breathing plain air, are normal in a fully recreational context, and are taught since the first-level course.
As said, it is not a small niche, it is the standard way of diving here.
The small niche is tech diving with accelerated deco (in pure oxygen or high-Nitrox), and perhaps trimix.
This requires additional training and difficult-to-find logistics for getting these special mixtures.