Unfairly charged for their delays, refuses to approve claim

My parents’ international flight got delayed and BA put us on a different flight (Qatar). I clearly asked about differences in baggage policy and they informed that we can expense it as its not our fault. Qatar charged us hefty for bags.

Cut to claims - they keep denying the claim. This cost us a lot and don’t ever recommend this airline. Poor customer service. Anyway, I’m thinking of going to Aviation Consumer Protection - any recommendations?