The conditions of carriage on the railways are not subject to unfair contract law, or it would seem most other consumer protection law. I’m not going to defend this, but that is the case.

Airlines are subject to UCTA and other consumer protection legislation, subject to a few exemptions that stem from things like the Warsaw/Montreal agreements.

BA have form for chasing a travel agent for facilitating ‘hidden city’ ticketing whilst simultaneously doing absolutely nothing to the passengers who did the booking and final sector dropping. This suggests to me:

1. BA really don’t like smart arses booking a cheaper itinerary and flying a more expensive subset of it -but-
2. They are not confident that this practice by customers is actionable

IANAL but I’m confident trying to force a mandatory reprice at a higher level for not consuming all the sectors on ticket is going to be deemed unfair. I know some people have suggested here it is dishonest and/or a breach of contract to (say) book ex-eu and intend to drop the final sector. To those people I’d point out it’s not dishonest if you genuinely believe the terms and conditions are illegal/unenforceable and there’s no obligation to actually fly whatsoever as part of any airline ticket sold in the UK. It’s an option to fly, not an obligation.