"If you’re not getting anywhere
You can report your issue to the Civil Aviation Authority (CAA) if the airline doesn’t give you what you’re entitled to.
PACT (the CAA's Passenger Advice and Complaints Team) will only consider your complaint if the airline or airport involved is not a member of an approved alternative dispute resolution (ADR) body."
https://www.citizensadvice.org.uk/consumer/holiday-cancellations-and-compensation/if-your-flights-delayed-or-cancelled/