Yet INV-LHR has done well and gained a night stopper against all expectations because INV has an APD exemption?
I think the APD exemption is only half the story. If what I hear is true, there is rather more "support" for this service than just the exemption from APD which is a matter of public record.
Can you point to any scientific evidence that APT affects flights? Other than whining airport and airline bosses?
The number of domestic air routes has fallen from 244 in 2008 to 190 in 2018, and a further eight have closed since then. The core of this fall has taken place on regional routes (ie outside London) where the number of routes has dropped from 194 to 148 and passenger numbers have fallen by 4% despite overall growth in aviation. The only meaningful train developments in that time have been WCML upgrades which have impacted on the likes of GLA/MAN. The rise in APD levels - and double taxation on domestic return journeys - caused demand to drop. Airlines then had to cut frequency, which further impacted demand, and then the air routes became increasingly unsustainable.
Some of the loss has come from closure of airports like Plymouth, Manston and Blackpool (to regular scheduled services). BOH has lost three of its four domestic links, BRS has lost seven, IOM eight, LBA nine - it's certainly a significant impact.