All simply explained here in the main thread https://www.flyertalk.com/forum/brit...thread-37.html

Book the outbound online the second the seats are released. Call when the return is released, again be in the call minutes before release, the agent will secure the seats, apply the voucher and take half the required Avios.

If you are Avios rich, you can book the return online using full Avios, then call the next morning to get half back and apply the voucher.