Sure. Being relatively crude (feel free to check the numbers), I make the population within 10 miles of one or more of these stations (in millions):
Euston/OOC: 7
Interchange/Curzon Street: 2
East Midlands: 1
Manchester, Manchester airport, Liverpool, Warrington: 4.3
Leeds: 1.3
Sheffield: 1
Glasgow: 1.3
Edinburgh: 0.6
Crewe, Stafford: 0.5
Preston to Carstairs: 0.9
Newcastle, Durham, Darlington, York: 1.9
Total: 21.8, of which 7 million are in London.
You could include all 9 million Greater London if you wanted given generally better public transport links but even then it's over 60% outside of London.
"Huge tracts" of England are relatively unimportant if nobody lives there. Half of the population of the UK as a whole lives within 25 miles of the HS2 route (just the high-speed bit, not including lines served by classic-compatible trains). A fast transpennine route is also important since there are so many people living on either side, but it's not an either/or thing.
Have a play around with the population mapping tool here:
https://sedac.ciesin.columbia.edu/mapping/popest/gpw-v4/