What routes though?
I mean, the vague OP sounds lovely in theory but which routes would provide anything like the benefits of HS2?
You mention Manchester - MML - London, so is that the line through Bakewell? Would that be requiring an additional pair of tracks from London to Derby to accommodate the increased frequency (given that there are already plenty of trains on the line, so many that the EMT services had to be reduced to accommodate all of the Thameslink trains)... how fast would you be building a line twisting through Bakewell so that it was competitive with the existing Manchester - London services? Or is this via the Hope Valley (where the freight already gets in the way of two and a half passenger services per hour but it might take five years just to redouble the short chord at Dore)? Where's the capacity at Piccadilly for all this? And the capacity at St Pancras?
Everything sounds good in theory, and would probably be popular amongst a nostalgic electorate, but eventually you get to a stage where you have to accept that (a) there's precious little spare capacity on existing lines and (b) it's much simpler to build brand new alignments than try tinkering with existing lines (look at how long the short bit of track at Dore is going to take).
Or would the idea be to provide an alternative to HS2 by focussing instead on lines like Aberystwyth - Carmarthen?