Was it a through booking on a BA issued ticket?

If so the CAA advises to process through the final carrier. But provides the option to demand redress from the ticketing carrier.

This makes absolute sense if you need to get litigious, as you are unlikely to get the final carrier to even respond other than through the subcontractor most airlines use to process such claims. Don’t lose redress to UK legal jurisdiction.

I still have an outstanding claim with AY from Jan 2023 where they delivered 24 hours late and with 3 bags completely wrecked. They insisted that MAN-HEL-SIN-KUL then MH were liable as final carrier. Quick research showed I had no chance whatsoever of getting MH to compensate. Ticket was AY UK and after 3 months they finally accepted liability - but made repeated stupid proposals - either for delay and asking for proof of baggage damage - and then for receipts for delay purchases (about £38) whilst they had had baggage damage pictures and valuations for over 6 months. Absolute shambles.

Point being don’t let liability jurisdiction get out of UK courts!

Understandably AY are now no longer an accepted carrier for companies under my transport approval authority.