https://personal.help.royalmail.com/app/answers/detail/a_id/5156/~/ive-received-someone-elses-mail
If you’ve received mail which has your address, but not your name, this is because we deliver to addresses rather than names. If this does happen, you can put a cross through the address and write 'Not known at this address' or 'No longer lives here' and put it back in a letterbox. Where possible, we’ll return the item to the sender hopefully allowing them to update their records.
If you’ve received a ‘Something for you’ card that’s not in your name, we recommend you dispose of the card and once our retention period has ended we'll return the item to the sender.
If you’ve received mail which isn’t in your name or your address, we're really sorry we've made this mistake and inconvenienced you. We'd like to get the item to the intended recipient as soon as possible and would greatly appreciate your help. Please post the item when you can, you won't need to apply any postage, we’ll then process and deliver it to the correct address.