I would look at using foamed glass for floor insulation, particularly if you are digging out as much of the foundation as you can and leaving the walls.
8 mph sounds about right, but I would need to do the maths. That’s why just going a bit faster, which you and the others weren’t, makes a huge difference.
OK. The only issue might be if/when you come to sell the property if the borehole isn't registered. Just heard that if the borehole is less than 15 metres deep it doesn't have to be registered.