@etchedpixels Curve radius also goes up with linespeed, so you have less flexibility with the route. As, of course, does energy consumption due to air resistance.
I would finish building HS2 to Manchester since the capacity is needed, the design work is done, and improvements around Manchester rely on it. But the proposed alignment is no longer protected, alas, so if I understand correctly there will need to be a new hybrid bill.
But once that is done, I think what the UK needs more than anything else is a rolling programme of infrastructure work, electrification and signalling modernisation, combined with incremental linespeed improvements. Basically what the TransPennine Route Upgrade is doing (and doing well), but fully funded and scoped in advance, and on every significant route nationwide.
No point having a shiny route from London to Glasgow if everything else is falling apart and your electric trains can't go anywhere.
@quixoticgeek