'Bullsh1t' I told them, you've got 24 hours to source a replacement aircraft, they didn't and having been rebooked on an alternative carrier, whereas the minimal load of 2 combined flights clearly indicated the, so called, 'technical' reason, and my suitcase having arrived in a absolutely destroyed condition BA told me it was that other carrier's problem, despite travelling on a BA ticket, and the other carrier having no representation in UK.
Nut Phileas did travel with BA. That is who he paid for his ticket and that is who his contract was with. If BA wishes to sub-contract the flight to someone else then fine - but Phileas' contract remains with BA and it should be up to BA to recompense Phileas and then claim it back from their sub-contractors.
If the airline terminates the contract then you would need to consult
the conditions of carriage to understand what terms you accepted when you entered into the contract. In addition there are statutory conditions that might apply in your case. For example EU denied boarding compensation rules. The airline would certainly be required to refund the monies you had paid for the ticket in addition to any statutory compensation. However if a passenger is re-booked on another carrier at their individual request, then the ticket is endorsed to the new carrier. This permits the new carrier to receive consideration for the transportation. This is quite distinct from the situation where the original airline sub contracts another airline to operate its own service.
If the ticket is endorsed to another carrier and you accept that endorsement, then your contract is with the new carrier who will accept responsiblity for you in accordance with their own
conditions of carriage.