I'm about to enquire about my situation with BA and have found this, which confirms the first sentence in the Thread Title:
https://naprepin.custhelp.com/app/an...way-journey%3F
Flights listed on your e-ticket can't be travelled on out of sequence, therefore once an e-ticket has been purchased as a return journey it can't be changed to a one-way, nor can it be used for a one-way journey.
The flights that form your e-ticket/journey must be used in the correct sequence -if you don't travel on a flight it will unfortunately invalidate all remaining unused flights on your e-ticket.