You already have the answer there.

Covered for 5kg satchels and non-satchel express parcels. The tracking events don't prove recipient's address (partly because they only show to the distribution centre and partly because you hand write the address), so that is why the non-5kg satchels don't qualify.

There are ways to be covered apparently, but what is wrong with just using C&S? If somebody really wants it urgently, they really shouldn't be buying online. Or at least they should buy BINs with plenty of time to spare.