The £18 billion is from TFL, and covers surface access schemes likely to be needed to alleviate increased demand to the airport.
How much of that is something TFL would need to invest anyway should LHR growth remain static?
At best some INV locals can access LHR easier than ever, and vice versa. The route already exists between LGW and has done for years.
There's a tiny fraction of the long haul connectivity avaialble over LGW compared to that over LHR. It allows a one stop connection for business to the world, easyJet to LGW is a different market entirely. So "at best" a fair bit of money will be inbound to the Highlands from overseas, even at one flight per day. Inbound tourism is a big money earner in pasts of Scotland, so in terms of getting US or Japanese tourists to that part of the world, LHR gives options LGW can only dream about. You might turn your nose up at one flight a day but GLA sings the praises of one B757 per day, from EWR. As does BFS, because they know the sort of importance they have to the economy.
Leeds and Inverness seem rather pleased to have a Heathrow connection again, maybe they should just shoosh and be thankful for what they have with easyJet and London. Say to the wider world, "Scotland's not for you"?
Sheesh!