It is illegal to "delay the Royal Mail". By delay the law means wilfully taking any action that prevents a person's mail from reaching them in a timely manner. Mrs Nick's action in opening and failing to forward your post or to return it to the sender is illegal. It is arguable that as resident she may open any mail addressed to any person that arrives through her letter box but if it is not intended for her she should either forward it to the person named as the intended recipient, without delay, or return it to the Royal Mail giving the reason why the addressee can no longer receive it at that address. Simply writing "Gone Away" on the envelope and putting it in a post box is sufficient to comply with the law. She is under no obligation to forward it to you or to give your forwarding address to the Royal Mail (in fact the Royal Mail should legally refuse to accept a 'third party' instruction to redirect another person's mail) BUT she must return it to the Royal Mail who will then return it to the sender.
The Royal Mail is under an obligation to deliver to the address on the envelope not the named person ~ actually the Post Office Acts state that they must place it through the door or "delivery point" of the address shown even if they know the person no longer lives there ~ this rule may only be circumvented if they are in possession of a signed and valid "Redirection Notice" so for once the Postman is right to deliver the mail through the door of whatever address the sender has used, unless you have completed a "Redirection" instruction, paid the fee (making it a valid redirection) and waited the given period of time (which used to be two weeks notice) then you have a complaint against the Postman. If you redirect your mail for a period of six months and then let the redirection instruction lapse then the postman is obliged to deliver to your old address. He is not expected to "remember" that you have moved although in the old days we used to. I doubt that today with their ever changing staff that many postmen are aware who lives where, they simply work to the address.
Yes ~ I know the postal service has its faults but most are caused by people other than the guys who walk the streets. Unfortunately every organisation has its bad apples and those are the ones we hear about. Remember, until it gets into the hands of the postman who actually delivers to your house it is directed toward its final destination by machinery (using the post code). Nobody reads the address until the morning of the actual delivery and postal delivery staff are trained to read only the bit that matters ... the house name or number and the street name. It takes far too long to read the full address on everything they carry. It is when sorting his round into the order for his walk that the Postman should pick up on any redirection instructions.