I think that is very true, and it also works the other way for records coming into the UK.
Postal workers know the system and they know that if a packet is shown on the system to have left the UK then it can be misappropriated if it isn't tracked all the way. Similarly, if a packet is not shown as having arrived in the UK, it too can be misappropriated.
Someone has a Troy Dodds of mine that disappeared in this way. The tracking shows it leaving the states but never appearing in this country.
The international mail services available from the UK are somewhat ludicrous to be honest:
"International Signed For" - tracked to point of departure from UK then not tracked until delivered when a signature is obtained.
"Airsure" - tracked all the way then not signed for. Why not?
Neither service is guaranteed. So effectively the mail services are admitting that they cannot deliver reliably.