Details of the sale are here.

Terms and conditions
The discount applies to new bookings made via the Customer Contact Centre between 30/5/13 and 28/6/13 for travel between 1/6/13 and 18/12/13, bookings made online are excluded from the offer. Discount is applicable to British Airways flights in Economy class with a full Avios value of 40,000 or 50,000 Avios only (BA code shares and IAG carriers are excluded from the offer) The discount applies to Avios and Avios and money bookings only. Taxes, fees and charges are not included in the discount offer and must be paid in full by all passengers.