In response to the news that BA is restricting short-haul and domestic flights from Heathrow in the summer of 2022, a Fantasy Ferroviaire using existing infrastructure and capacity to provide some rail connections. Even without HS2, the relief lines of the Great Western and freight connections around London open up some possibilities.
City-to-city journeys are already possible by high-speed trains (and my EuroNight 6 https://fantasyferroviaire.wordpress.com/2022/01/10/en6-joining-the-dots/)- but since short-haul flights also act as feeders to long-haul, here are some direct rail connections between Heathrow and Birmingham (1h40), Paris (3h10), Manchester (3h15), Amsterdam (4h25) and Frankfurt (5h25) airports. Let the train take the strain.
(A spur from the Elizabeth Line to HS1 at Stratford would shave 30min off the continental timings)