Mikie 4 in a single work NO. Its been tried to Heathrow (Cambrian) and London City (Air Wales). The railway at the moment is faster to London than a flight can be. It will probably become even faster with electrification.
Xtype
Yes, and there will apparently be some direct (no changing at Reading) trains to/from LHR when the western link is opened.
That's why CWL on a map of new domestic connections in "Taking Britain Further" was a surprise to me.
Guess Heathrow Airport Ltd. know better.
Cambrian was swallowed up by BA, maybe those slots were diverted to long haul routes. LHR was declared full in the 1970s, so scarcity of slots is not a recent phenomenen, and thin domestic routes were the casualties.