I think you will be fine. In a situation like this your booking is effectively split with the outbound ticketed and the inbound not, pending it being reissued. If you have the BA app then you should still see ticket numbers against the outbound flights. Alternatively if you have the BA PNR then you can look on classic.checkmytrip.com and you should see the ticket number there. If you don't see the ticket number and can't OLCI then you would need to ring BA and they should reissue it while you are on the phone.