To knock down a burnt out shell of your house, then get the foundations inspected then re build your house to the spec it was using a principal contractor to do the whole thing will be £700,000 all day long.
just because your going to build it cheaper using your head and a bit of carful planning doesn’t mean the insurance company can do it like that.
if you suffer a total loss you don’t want to be under insured.
have you had a quote to build the whole thing with zero involvement from you, it bet it will be £600,000.