I read the Spanish Court's decision as you can use any part of a ticket and skip the others without giving a reason. Unless this question goes before the ECJ then it looks like it only applies to IB.
https://www.lexology.com/library/det...9-3d02e394cb38
Which? is campaigning against airlines that cancel return portions of the ticket when the outbound was skipped due to circumstances outside the passenger's control:
https://www.which.co.uk/news/2018/12...-show-clauses/, however BA's Ts & Cs already allow for protection on these grounds as quoted in the OP.