I don't see why you should be reaching for your tin hat A4. Most prudent holiday makers, when booking, check to see who they are flying with. Usually the brochure has towards it's rear, the airline to be used on a particular flight plus the type of aircraft to be used. O.K. so the operators always put in the saving clause that they may substitute other airlines and aircraft. However it is a big selling point that you will be travelling on a British Airline.
I too would be miffed to find that the aircraft that turned up would be on a sub charter to a sub charter and I would be extremely dubious about the quality of the crew and the aircraft's airworthiness. I stopped using one holiday company when instead of the nice shining B737 operated by a British company I was expecting to fly in, an ancient 727 operated by a Portugese airline with only 2 aircraft turned up at Manchester 24 hours late.