Most of the postal services act is aimed at Royal Mail workers. Only those working at the Returned Letter Centre, for example, can open post, and then only to try and establish an address to get it to. Post ends up there when either the address is incomplete/unclear/doesn’t exist or it’s been returned to sender and there is no clear return address on the outside. Those who sort post via MDEC have three seconds to sort each piece of mail, after that you are deemed to be holding up the Mail, another offence. Although you’re likely to get a rollicking rather than arrested! As an aside, if you ever think you have a boring job, work for Royal Mail on MDEC. Mind-numbing doesn’t even begin to describe it!
Royal Mail consider the job done if there is an address on it and it’s delivered to that address, the name on the post is of no relevance. Indeed postmen will often have to post something through a door knowing full well that the person doesn’t live there, but they have to deliver as addressed.
Obviously, they are only human, sometimes it gets misdelivered. You can call Royal Mail to complain, the postman will be spoken to (it’s a serious offence) which is why they will need the date it was delivered. You may even get a book of stamps by way of apology-but that’s less likely now, certainly for a first failure, than it was.
If you return an item, cross through the address, write “RTS” on it, and if it’s unopened it’s free. If you have opened it, in theory at least, you need to put a stamp on it. Of course, if it’s a simple misdelivery either pop it through the correct door or back in a postbox if unopened.
Certainly, if an item of mail is delivered to your address and that is the address on it, you’re not committing an offence by opening it, only if having opened it you use any information contained in it for nefarious purposes are you breaking the law.