You can book the return online as well as long as you have sufficient avios and then phone up to get the voucher applied to the return and get the avios refunded. Although I did read somewhere ( probably on this thread ) BA were implementing a change fee for this now I think. Not sure if that is still the case.

I’ve always booked both ways online but on the stroke of midnight and phoned up after booking the return. Last time was 11 months ago though.