British Airways has extended its seat sale to December 12, offering roundtrip sale fares in World Traveller economy from $840, including all taxes and fees.Customers can travel outbound until December 16, 2012 and then again from December 24 to March 24, 2013 giving them the opportunity to visit the UK over the holiday period and into the spring.Under the terms of the sale, fares in World Traveller must be booked seven days in advance.Weekend travel is an additional $40 found trip. Minimum stay is Saturday night and changes can be made for $275. Seats are subject to availability.Tickets can be booked through local travel agents, by phoning the toll-free number, 1 800 AIRWAYS, or by visiting the website at www.ba.com.