It matters greatly that an item can be proven to have been sent - most British law that makes reference to sending documents relies on The Interpretation Act 1978...
s(7) References to service by post. E+W+S+N.I..
Where an Act authorises or requires any document to be served by post (whether the expression “serve” or the expression “give” or “send” or any other expression is used) then, unless the contrary intention appears, the service is deemed to be effected by properly addressing, pre-paying and posting a letter containing the document and, unless the contrary is proved, to have been effected at the time at which the letter would be delivered in the ordinary course of post.
...thus a properly addressed, prepaid letter entrusted to Royal Mail or other carrier is legally deemed as having been sent; non-delivery does not matter unless you can prove it was never delivered and how do you prove a negative?