From the conditions of carriage:
3b) Ticket validity period
3b1) Unless it says differently on the ticket, in these conditions of carriage, or in any tariffs which apply, a ticket is valid for travel for:
•one year from the date it is issued or
•one year from the date you first travelled using the ticket, as long as your first flight took place within a year of the ticket being issued.
http://www.britishairways.com/en-gb/...ns-of-carriage