IMHO, the idea of building a UK Shinkansen was flawed from the start.
First, it is very doubtful the Japanese would have built it that way originally if they had had a classic standard gauge network.
The proposals for Shinkansen style lines predate the decision to build at standard gauge however.
It's a concept that dates originally from before WW2, so I am a little skeptical on that score.
Second, although the population density between England and Honshu are comparable, in other ways they are not. Most notably, cities in Honshu are much bigger, if you ignore Tokyo/London - meaning that England is more densely settled outside of the main cities. As such, a network only based on the largest cities was bound to have much less advantages for the population at large than in Japan.
But a Shinkansen style solution would not be based solely on the main cities? Indeed that is the central characteristic of Shinkansen.
An M1 Shinkansen would likely go London-Luton-Milton Keynes-Northampton-Coventry-BIrmingham
The problem is that HS2, as currently conceived, doesn't really reduce the operating expenses of the WCML by very much.
It would make a great deal of sense if our rail service patterns were like Europe with a clear delineation between "Intercity" and "Local". The Intercity trains would be removed and their costs transferred to the new lines, and the local service would continue as before.
But British rail services aren't like that, so HS2 is unlikely to allow many services (and thus costs) to be removed from the WCML because the intermediate destinations all have to be served regardless and the government is unwilling to make the sort of swingeing service cuts required.
If we had a Shinkansen, there would be little need for anything but a small local service north of Wolverton/MKC, which could be served by extending the Tring terminators. Virtually everything else would have moved to the high speed line.
But this is all wandering rather far off topic! So I suggest we either create another thread or park it.