You must book by 31st December using discount code OPODO20.
There are no other BA or Virgin discounts at present for long-hauls from Heathrow, so this is as good as it gets.
Note that BA flights booked through Opodo incur an £3.70 per person, per segment (ie £7.40 per person return) booking fee. There is no fee for booking Virgin Atlantic flights as far as I can see, so this is a far better deal when booking with them.
There is a credit card fee of 2.5% (so using an Amex will cost more than if booking via ba.com – debit cards are free, however).
Don’t forget the two hotels sales currently running as well – 33% off Hilton weekend nights for the whole of 2014 and 30% off Radisson / 50% off Park Inn for weekends until August 2014.