How many former Members of the House of Commons still hold a post box within the building and at what is the annual cost to maintain?
No former Members of the House of Commons still hold a post box within the building.
A mail redirection service is provided for defeated (from either a by-election or General Election) or deceased Members for a period of three months, the cost of which is met by the House of Commons. After that time post is directed to the new Member returned for the constituency. If the Member wished for mail to be forwarded beyond the 13 weeks they would set up a redirection of mail with Royal Mail and at a cost to themselves.
Separate arrangements are in place for former Prime Ministers (who are not members of the House of Lords); costs are not met by the House of Commons and so the information is not held.