In Japan you pay a premium every time you want to go on an express train - be that the Shinkansen - or a rural express. Doesn't deter anyone as the journey time is so much shorter.
The only way to interchange HS2 with HS1 at St Pancras would be to tunnel HS2 deep down below the underground lines at presumably prohibitive cost, as the area is so congested. Aklternatively they could bulldoze a large residential area.
It took Tokyo years and years to build a connection in central Tokyo between the Shinkansen coming from the north to those going to the south and even now all the trains still just terminate in Tokyo. Most people travelling down HS2 won't want to go any further than London.
The only way you can really do joined-up transport policy is to create a brand new capital somewhere...