Hi crewmeal
Your thinking is spot on, but why should I fly from ALC to MAD to LGW when I can fly direct from ALC to LGW with EZY, who, in my 10 years of flying with them have never cancelled a flight, apart from the most severe circumstances?
If I lived in Glasgow, and wanted to fly almost anywhere in the world, the first step would be to fly to LGW with EZY. Once at LGW I can pick where I want to fly - this is the market that BA need to tap into.