When I was involved in the work, ahem, 15 years ago this very month, we reckoned it was just possible to do a 2h30 time to each of Edinburgh and Glasgow with a 350km/h HSL throughout, allowing for 2 stops (at putative stations close to each of Birmingham International and Manchester Airport). Little better than crayoning at that stage, but it helped demonstrate the possibility of very significant journey time reductions.
With those times, you’d capture something like 75% of the air market. The rest would be that which is connecting through Heathrow to long haul (or minutely but importantly, at Glasgow / Edinburgh for the isles), plus traffic which as you say have an origin / destination point convenient for the airport.
There’s a ‘but’. Unlike the rest of the air market, London to Central belt has barely grown since then, as the new Virgin timetable, improvements on the ECML, and the changes to air security procedures have led to rail capturing all the growth in the market. Nevertheless, if 75% of the air market swapped to a high speed line, it would be worth about half a billion a year in income. But about half of that will come with HS2 phase 2b anyway.