The key part from the website was posted earlier and ignored.
"We don't charge for postage, handling or any transaction fees, but if your device(s) are of value, we recommend you take the extra step of paying to track and insure your package using Special Delivery guaranteed service, available at all Post Offices®. "
Seems simple to me reading that. If the phone was worth less than £40 a certificate of postage could be obtained for free, or if a signature was required then recorded delivery.
If the phone is worth considerably more than £40 then Special Delivery should be used as in the event of the phone being damaged, lost or the third option that is being implied the standard service only covers you for £40 so special delivery covers the extra value of the item.