And, perhaps, a final reason is that Madrid is one hour ahead of London - which makes it even more difficult to have a flight leaving the East Coast and arriving at a sensible time in MAD, because the nominal flight time is an hour longer.

So, a combination of longer routing, an extra hour lost through time changes, limited demand on the route in the first place and inefficiency of aircraft usage means it ain't gonna happen.

It really only happens into London - if it were to spread elsewhere, MAD would probably not be the first. It's a shame as I always take the day flight if I can - much less jetlag.