'If you don't redirect your mail, anyone could be reading it'. It might seem like quite a lot of money to mainly get your Boden catalogue but it is surprising how many people don't bother to redirect their mail. Random people you don't know could have access to all sorts of information about you if you leave your post at an old address ... your bank statements for example. Walks in our office can have anything from four or five to 40+ redirections. They are a physical thing (as I tried to explain to a customer once), a plastic wallet containing the instruction, with the old and new addresses, and stickers with the new address on. Each day the sorted mail is checked for each redirection and letters for the specified people are stickered and sent on. The parcel scanning system will also flash an alert on tracked parcels for any address that has an active redirection. I know you have a lot to think about when you're moving, but in effect this can save you trouble as it gives you extra time to inform all the people who need to know that you are in a new place.
(Disclaimer: this A to Z is not official Royal Mail advice, except by coincidence.)
Stay safe. Be kind. Protect your identity.