Isn’t the part of the US where Ray’s park is under 4ft of snow for a few months at a time? Thats probably why that works (although I’d seen the videos and to be honest, it’d bore me silly riding around at walking pace in a warehouse).
A friend of mine looked at doing something like it in the UK about 10/15 years ago and the maths just didn’t work out.
Dirt Factory in Manchester I went to a couple of times and actually thought it was pretty good with pretty limited space but looking at their website it looks like they’re currently homeless (?) and looking for bigger premises (or at least thats what the last update nearly 18 months ago was saying). That was definitely more jump orientated as well rather than “trails”.
I think with the rise of the trail centre in the UK, theres no need for indoor biking as they offer an “all weather” riding option. Yes you might get wet, thats what waterproofs are for.