Originally Posted by Often1

In order to prevail, OP would need to proceed against BA, the operating carrier, for a departure xUK and assert that Spanish law as to IB's COC applies. If a claim under EC 261/2004, that might result in compensation of as much as EUR 600 and presumably the cost of a new ticket to replace the cancelled ticket. If not, it is unclear. BA would, of course, assert that it was advised by IB that the ticket was cancelled.

One would have to lay out a lot of money against a speculative return of some or all of it at some point in the future.

For what it is worth, when Germany did roughly the same thing, the "ill" was cured by carriers offering tickets which need not be flown in segment order. They are, however, vastly more expensive. A phyrric victory at best.