Japan built the first Shinkansen while British Rail was still running steam services. Can't stay on the Victorian era rail constraints forever.
(it's very British to say "this is too good, can we have something cheap and nasty instead please?")
(it's very British to say "this is too good, can we have something cheap and nasty instead please?")
Quite a lot of the cost is the NIMBY appeasement mentioned upthread. Something like a quarter of the line will be in tunnels. Making a slower line wouldn't make that any cheaper.
The extension to Euston was supposed to have 11 platforms. Even the reduced scope now being implemented is 6 platforms, I believe. All 11 were required to handle the eastern leg of HS2 [providing bypass capacity for the East Coast Main Line out of King's Cross and the Midland Main Line out of St Pancras], and services to Scotland and Manchester [bypassing the West Coast Main Line from Euston's classic platforms].