I can see merit in rigorous testing of the home educated, but I am alarmed at the suggestion of government set texts.

Somebody brought up the New Zealand example. That's exactly what they do (or did when I was there). The state operated a primary education system for kids living on remote farms. It used a curriculum that was, if anything, more fixed than the one followed by kids in ordinary schools. Packets of material sent out by mail and teachers following how the kids were doing, also by mail. It worked very well; you couldn't tell kids educated that way apart from others, except that they tended to know a lot more about the sexual physiology of farm animals and had peculiar gaps in their knowledge of what you could find in a city. That sort of system doesn't have anything in common with what UK homeschoolers want. Hill farmers don't have time to be neurotic weirdos.