On Monday, 21 November 2016 10:34:58 UTC, The Natural Philosopher wrote:
An elderly friend is going abroad over Christmas for 5 weeks to visit
family, and wants to know what to do about his letterbox, which wont
hold 5 weeks of mail and would rather give the game away if left stuffed
with weeks old letters.

Royal Mail Keepsafe stores incoming mail for up to 66 days.
http://www.royalmail.com/personal/re...-mail/keepsafe

£23 for up to 31 days, £44 for up to 66 days.

Poste Restante seems to only hold mail for two weeks.

Poste Restante is for travellers on the move needing a temporary address.

Owain