@witheringrowan
Full capacity of Eurostar trains is about 850 passengers. I don't know how that compares to a plane.
For info for your common or garden short haul aircraft such a a 737 or an A320, think in terms of 180 per aircraft, but there's a big plus/minus on that depending on variant.
But because of the issues I mentioned before, they could only run 2 trains an hour - and that has to cover Paris/Brussels/Amsterdam. I think at the height of the problem capacity was down 30%, although it has eased slightly now. I don't know how many flights depart all London airports every hour for those destinations, but I'm happy betting its considerably higher.
Haven't got time to dig into now but top of the head you'd quite possibly looking at (again on average) one an hour out of somewhere like Heathrow to Paris, a bit less frequency for Brussels and Amsterdam...then of course as you say, factor in the other London airports, plus flights from the UK regions directly to those three destinations..(e.g Manchester - Paris)
Not sure what these subsidies on flights are that have been mention, always seemed like the money went the other way, for example Airport Passenger Duty (APD) which people pay on their tickets makes HMG a significant amount of dosh in a year...(over 3 Billion sterling in 22/23).
If people are thinking of the old chestnut of (no) tax on fuel well, unless you can reach an agreement to equalise any such tax across, say, the whole of the EU you're almost certainly doomed to see something called tankering increase on some routes, which would increase emissions
https://www.icao.int/environmental-protection/Documents/EnvironmentalReports/2022/ENVReport2022_Art38.pdf