I think point 2 will change once this becomes common. Once there is demand Home Depot etc are the perfect place to sell individual medium sized panels.
Hopefully it won't take 10 years for this to become easy.
The UK has quite a few kinds of "Do It Yourself" stores, from somewhere like B&Q that is catering primarily for consumers through to a Screwfix who are more trade focused but still open to the public.
This particular niche is one where the UK did a lot of the US-style "Business is just a large metal shed, surround it with car parking, build it miles from anywhere" whereas for a lot of other shops the UK doesn't do that, preferring to put them together in walkable parts of a city or town instead. DIY stores are just especially suited to huge metal sheds in a way that furniture stores or grocery stores are not (those do both exist in the form of giant metal sheds in the UK but it's less common) - and their customers typically drive.