The correct decision is to route HS2 via Heathrow and offer protected connectivity onto trains to/from Manchester (and then ban UK domestic flights on London-Manchester - later same with Leeds, Newcastle, Glasgow, Edinburgh), not the other way round.
It might work with New York - Manchester for a few foolhardy punters but most who want to go to London will still fly to London. And when it comes to hundreds of other smaller markets, there’s just no additional demand to the north that will fill flights to Manchester in preference to London.
I wholeheartedly agree with banning domestic flights within the UK, especially after HS2 to Manchester opens.
Most people from New York who wish to travel to London will indeed want to take the quickest route but you have to bear in mind that as another member posted upthread, once HS2 phase 2 opens, it will become quicker to get to some parts of Central London from Manchester and Birmingham airports than it will from Heathrow. Most people aren’t going to believe this unless you tell them because they assume any airport with London in the name means it serves that city, while anywhere without it is nowhere near.
I’m not saying Birmingham and Manchester airports should have London included in the name, but I think it would be good if a system was in place that meant all three airports get a fair share of traffic depending on their capacity.
For example, say that there are 20 flights per day between New York and London.
Under my system, Heathrow should get 60% of the UK’s long haul flights, Manchester gets 30% and Birmingham gets 10% therefore these flights would be split between the three airports, so LHR gets 12, MAN gets 6 and BHM gets 2. The same principle would apply to flights from other international destinations.
Under my proposal, the three airports would essentially act as mega terminals for a 5-runway hub spread across the country, linked together by a 200mph railway. Once that is in place, all you need to do is heavily promote and market Manchester and Birmingham airports as being an hour away from London and hey presto, you have your modal shift!