As far as I can find, Gatwick has slightly over a third of Inverness's domestic passengers (700,000 in 2022); I don't know how many go to Heathrow or Luton, Amsterdam (the only year-round international destination) or Bergen or Palma.
The ORR statistics say 15,000 annual passengers between Inverness and London; the next highest south of the Central Belt are Newcastle (6400), York (2600) and Darlington (600). Split ticketing is almost certainly involved there though; I struggle to believe that there's more demand between Inverness and Darlington than Leeds.