I'm not aware that the answer to this question can be found in Southern Railways published Terms and Conditions.
However other bodies, including statutory bodies, will accept a recent landline telephone bill as identification of address.
Rather than take comfort from my interpretation alone, I suggest that you also take additional evidence if you can, such as a bank or credit card statement, contractor's invoice, HMRC Statement, Council Tax assessment, National Insurance reminder, Television Licence, NHS appointment, Mortgage statement, Pension contributions statement - I'm sure you must have something from an official body in the past year with your address?
Hypothetical question, but quite important. Does anyone know what would happen if I were to miss the last train to Milton Keynes due to a late running connection? Would it be possible to have the ticket endorsed to use London Midland?
I have allowed 45 minutes at Clapham Junction to grab a snack and allow for potential delays, but it'd be nice to have an idea of what could happen.
Also, I assume the ticket is eligible for Delay Repay if a delay occurs anytime during the day?