OK I used to regularly commute from Inverness to Bristol by easyjet and there is no way you could arrive at the airport at T10 and be taking off at T35. And those are small airports, not Manchester or Heathrow. So I am sceptical about some of your timings.
My best was 3 minutes taxi to gate 141 (assuming hand bagage only and already have boarding pass). Gate closes at T-10, so 15 minutes for taxi to gate is ample. I normally get there about T-30 and have time for a quick drink in the lounge.
Heathrow is longer, you have to be out of the taxi at T5 at T-37, although at T2 I've enterred security at T-18 in the past and made a domestic flight without issue.
I'd rather not cut it so fine, T-45 and T-30 respectively is better.
HS trains between Brussels and Paris have destroyed the air market between those two cities, despite Paris being a major hub airport.
Brussels isn't a hub, so it's only getting O&D traffic and people connecting through Heathrow.
On Monday there are the following flights to the 5 London airport
EDI: 28
GLA: 21
MAN: 9
NCL: 7
LBA: 3
BRU: 10
CDG/ORY: 18
If you had a
2 (not 3) hour train from Edinburgh you
may drop the 8 flights into London City, that would leave you with 20, which is a similar amount to Paris.
I don't see any reason why it can't happen in the UK. There may always be a handful of connecting flights but the vast majority will go. I also suspect that major airlines like BA would offer through plane/train tickets for international travel. This would then free up some landing slots which could be used for more lucrative long distance flights.
BA don't run those planes for O&D flights (at least the Heathrow ones), they run them for the connections.
There are 2 flights a day from Exeter to London City, for O&D traffic. Exeter is tiny and it only 2 to 2.5 hours from London on train, yet sustains direct flights to the city. If Exeter can, even with a 2.5 hour Edinburgh/Glasgow run I'd still expect 5 or 6 flights each to LCY.
As for a Lufthansa style train integration, Virgin don't offer through tickets, and sadly they failed with little red - not enough transfer traffic. I expect if domestic flights go, the majority of passengers will simply transfer to. I already do this east, flying Qatar via Doha rather than BA via London, and my last trip to the States was on US Airways.
But all this ignores the elephant in the room. If HS2 was actually a hyperloop and did Glagsow/Edinburgh/Manchester/Newcastle/Leeds-Heathrow in 20 minutes, it will cause more emissions from Heathrow flights.
Remove those 37 flights from Heathrow and you'll just add 37 long haul flights. A long haul flight causes more pollution than a short haul flight.
If you want to reduce pollution caused by planes flying to/from Heathrow, increase the number of domestic flights. This would reduce the long haul slots dramatically.