If you are unable to return your keys to the office, they can be returned by a trusted person. If you need to post them, you can do this, but you need to take into consideration that there is a chance your keys will not be received by Bron Afon after your end of contract date.
Please bear in mind when using this method or trusting another person as you may be charged rent if your keys are received after your end of contract date or not at all.