Absolutely, there are ways. However some/all of them could have already been done and haven’t. With through tickets it would be easy enough for GWR. Why didn’t Virgin do this when they had the WCML franchise?
There’s a failure to recognise that there might be an issue (gettaefalkirk) and not everyone will/can go via train via OOC, even if it’s cheaper/quicker.
Re the last point I do disagree though. Much better to entertain in an airport lounge (with your bags held by your carrier), where you have more space (and at Heathrow, things to do) than train/plane, with multiple transfers and on Heathrow-OOC leg no guarantee of getting seats etc, likewise if you miss the right HS2 train due to a late flight.
Part of the reason that is not been done before is that the numbers who couldn't get a reasonable flight had been small.
With a fall in the number of regional flights, with more of the country wishing to use rail, potentially even a lot fewer people flying overall and so on; then the need to make changes and not carry on doing what was done in the past becomes more important.
If you can charge someone only slightly more than it would cost then to get to their nearest airport but make them go to the major hub airport by train then not only have you saved the cost of running those regional flights but not increased your passengers costs then that's going to make it attractive to some passengers.
At the moment there's a need to run regional flights between London/South East and Manchester, once you impact further on those flows then they are likely to be cut like the London and Birmingham flights have been.
The smaller the network of flights that you've got to run then the more expensive it is to carry each passenger on the remaining flights. Especially if you have lost passengers to rail anyway and you aren't running as many flights or the flights are being run with smaller aircraft on the remaining routes.
The other factor which is likely to come in to play is carbon emissions, there's going to come a point when air is still burning a lot of fuel to move the aircraft around, to hit carbon neutral (even if you ignore international flights) then there's going to be a need for fewer people to fly internally. With those who still do do will then need to carbon offset for their flights, that's going to make flying even more expensive.
The guidance is that each person should produce 0.6 tonnes of emissions per year, that's comparable to one of:
10,000 miles by EV (current energy generation)
3,300 miles by petrol/diesel cars
10,000 miles by train (current diesel/electric mix)
2 return flights from Glasgow to London
(That assumes that you create zero emissions by any other source, however EV and rail are likely to improve over time due to fewer diesel trains and more green energy. It also doesn't account for infrastructure emissions which are worse for roads than rail, with rail being about 40% of that of road per mile. Nor does it take into account manufacturer of the vehicles, with batteries being about double that of ICE vehicles, something which electric trains generally don't have a problem with).
I wouldn't be surprised if we started to see personal carbon credits being brought in, with people able to buy additional credits by carbon offsetting and selling credits if they don't use their full allowance.
That would have a significant impact on the numbers wishing to fly, unless aircraft are able to go electric (even hydrogen isn't great). However even if they do the power requirements for getting then airborne is going to be significant.
The graph below compares rail's fuel source and the emissions created, as you can see even just on traction batteries are worse than using OHLE, with Hydrogen being worse again:
It is why hydrogen is only ever going to be part, and probably a fairly small part at that, of the powering of rail.
As such, even though using hydrogen to power aircraft would be better than the current situation is still going to be worse than using rail.
The other thing to consider would be, without the need for so many airports, they would be an ideal source of brownfield development sites.
On the flying/train with children, I know someone who is a flying nut. Loves all things flying, wanted to be a pilot, can identify aircraft from the ground, etc. When they and their family go to Paris (last time for Euro Disney with 2 under 6's) guess how they got there? It was by Eurostar (including traveling for an hour by train into and across London) as they find it less stressful than flying. OK that is worth two adults, but they don't use rail week in week out (maybe about a dozen times a year) and would have to navigate the tube.
They are on a booked service, so have to get there on time, are fairly confident at traveling by rail but not all the time travelers, they have small children and luggage and are needing to navigate across Central London. I suspect that they would be fairly typical of those wishing to use HS2 and as such Old Oak Common would be much easier to navigate than the tube network. As such a fair number of people would be willing to make the change if there was a bit of extra help, which is what would likely happen if there was a plus rail app.