>>2045366Pic is $8,500 at Lowe’s (huge home improvement retail chain in the US). The price includes assembly and delivery within however many miles. They probably don’t meet building codes but who’s gonna know?
This kit is 20’ x 12’, and includes everything you see as well as a subfloor. There are other kits and other brands, but this gives you an idea.
Finish work would have to be done by you, as well as electrical, plumbing, and insulation. The costs add up, and it’s cheaper to frame a small house yourself, but this is much faster and much easier. You’re really paying for the convenience.
Not sure if there’s a British equivalent.