Back in 2010 BA called me because I did not fly the return leg of LHR-GOT-LHR.
I explained what happened (last minute change of plan which involved another return ticket with BA) and the operator told me "that is fine, for this time we let it go but please avoid to do no show and always call us or your travel agent to sort it out if your plans change".
They were considering to change the ticket to o/w and charge the difference.
U!