Yes you can do this - as per the main thread, and also yesterday's thread
https://www.flyertalk.com/forum/brit...241-query.html
https://www.flyertalk.com/forum/brit...iscussion.html
Q: Can I book an open-jaw trip using the voucher?
A: You can, but you will need to call BA. For the trip to be valid, the length of the surface sector must be shorter than either of the flown journeys. If you want to fly from LHR to AAA and from BBB back to LHR, then the LHR-AAA and BBB-LHR distances must both be greater than the distance between AAA and BBB.