Sensible words and I don’t contest any of it.
I suspect (with no evidence whatsoever) that the % of passengers from London airports using Schiphol for transfer is rather lower than from U.K. regional airports, purely because London airports collectively have a much better range of long haul destinations.
I suspect that you're right that interlines from Heathrow and City through Schiphol are a bit lower than from the UK regions. However, if it is true, then I'd also speculate that this is down to the strongest origin and destination (point to point) market being beteen London and Amsterdam - and (particularly business/frequent) travellers will favour their home airline and its loyalty programme for the journey.
As an example, seven and a half returns in Club Europe to Amsterdam gets British Airways Executive Club members Silver privileges for the remainder of their membership year and the following one.
That's free seat allocations (including some extra legroom etc.,) at time of booking, increased Avios earning, Business Class lounge access when travelling in Economy or Premium Economy at other times, increased checked baggage allowance, fast track security etc., etc. So, if you're generally a BA or other
oneworld airline customer, then you'd likely make your Amsterdam trip travel mode choice with at least that in mind.
A similar dynamic will be at play if you're a 'loyal' KLM or SkyTeam member airline frequent flyer, so this would shunt up the O&D/P2P demand.
However, to a greater or lesser extent, it'll be fare and fare agreements that drive it; BA competes with aggressive fares out of Amsterdam, and KLM does exactly the same out of London.
As examples:
Economy Class London Heathrow to Cape Town, 2nd to 9th Novermber - £525 (Air France KLM) or £686 (British Airways)
Business Class London Heathrow to Cape Town, 2nd to 9th Novermber - £2318 (Air France KLM) or £2935 (British Airways)
Economy Class Amsterdam to Cape Town, 2nd to 9th Novermber - £640 (Air France KLM) or £615 (British Airways)
Business Class Amsterdam to Cape Town, 2nd to 9th Novermber - £3050 (Air France KLM) or £2116 (British Airways)
So, as I say, whilst I agree that there's likely to be a depressive effect on interlining through Heathrow from Amsterdam on BA, and through Amsterdam from Heathrow on KLM because of the availability of direct flights to the final destination from both, and a heightened demand from O&D passengers, it can still be upscuttled to a greater or lesser extent by the fare structure that the airlines employ.
And this, of course, is what British Airways will continue to be able to do, all things being equal, with fares from Scottish Airports as part of their overall revenue management strategy for their network, including UK domestic flights, whether the ECML and WCML went to 15 minute frequencies, or HS2 cut journey times, or indeed if easyJet acquired Heathrow slots. It's a deregulated market, so the airlines have huge flexibility in terms of pricing, and fairly low fixed costs.
