If the Royal Mail lose your parcel the compensation due would depend on how you sent the item, if you just paid for postage, then you’ll get the value of the postage back.
If the item was of significant value, and you only paid postage, they will generally compensate you to a maximum of £20. You will need to provide some evidence of what was in the parcel.
If you’re sending something that is worth significantly more than £20 then you really need to insure the delivery and provide evidence of the parcel contents.
Details of insurance prices and guarantees are available on the Royal Mail’s website
Royal Mail’s retail compensation policy for lost items