Quoting from the Act.
Interfering with the mail: postal operators
(1) A person who is engaged in the business of a postal operator commits an offence if, contrary to his duty and without reasonable excuse, he
(a) intentionally delays or opens a postal packet in the course of its transmission by post,
Now, I was chatting with a friend who works at my local sorting office today and the conversation got onto first and second class mail.
Apparently, if you were to post two identical letters, at the same time, to the same address, one with first class stamps and one with second class stamps, they would receive exactly the same service from the time they were collected from the positing box until they arrived for sorting at the delivery sorting office.
At this time, the first class letter would be placed in the bag for the appropriate delivery walk for the next available delivery round.
The second class letter would be placed in exactly the same sort of bag, but destined to be delivered three or four days hence.