FDF - wrong to say that Tokyo has just one terminal. It has many - Shinjuku (a massive interchange), Ikebukuro, Ueno (where all the Shinkansens from the north used to terminate until the connector to Tokyo was built, many years later) for example.
What Japan does have is effectively a multitude of Crossrails as tube lines link up with private suburban railways.
Also Tokyo is double the size of London, japan double the size of UK.
And most people on HS2 would be heading for London and not beyond. That's where the bulk of the market has always been.