I work in a Post Office so maybe I can give you some correct information. If you did not pay for it to be tracked (using Airsure or the much more expensive Parcelforce International), which I'm assuming as it's a calendar you didn't then there will be no way of knowing what happened to your item unfortunately. All sorts of things can happen, from the wrapping not being stuck down properly and it getting caught on something and ripping, thereby the address becoming separated from the item, to Royal Mail suspecting that it contains a dangerous item (details from your local Post Office) and destroying it. You would only find out if that happened if you put a return address on.
Anyway, you can claim for the cost of the item even if you do not have a certificate of postage, though it does make easier to claim if you do have one. Just pop into your local Post Office and ask them for a claim form.
It is annoying when things do not arrive at their intended desitination, but no system is 100 % perfect.