Well the OP did say:
(Emphasis added)
And until (

) Boris get's his bridge I think we can safely assume that there are not going to be adequate rail connections between NI and the rest of the UK!
I think we should certainly be considering it but it would probably only be a handful of routes at the present time. Personally I see no reason why we shouldn't strongly discourage/outright ban domestic flights between
London and Birmingham, Bristol, Cardiff, Manchester, Leeds, Teesside and Newcastle for instance. All the relevant conurbations have fast frequent services that link them. I would say that feeder flights for international travel should remain (and I believe that is the approach being taken in France and Austria).
But for London to Scotland, for now, I would suggest the journey time isn't quite there to make it reasonable to remove the element of choice for now. But post HS2 that will certainly need reconsidering. I also think that there are certain flows where it will always be necessary to maintain domestic flights. Someone wanting to Inverness to London for instance. Whilst it has a reasonable connections considering the distance rail is not a massively competitive mode for that journey.
Broadly I think it's something worth exploring but only on a handful of routes (primarily radiating from London) and only if the ability to make connections is maintained (so you could fly Leeds - Heathrow - New York for example still).