There was a time that Brit ACMI operators were available to fill these gaps. Sadly most have gone, unable to compete on a level field with operators from European countries with a much cheaper workforce and overheads. I have to say that if I specifically booked to fly on Monarch, Thomas Cook, Thomson etc., I would be seriously miffed if I found I was being caried by an operator which was not of my choosing. Legislation should at least protect the consumer with the right to cancel and obtain a full refund if the aircraft provided is not that of the company selected at booking.