The comparison of HS2 with 18 paths per hour in each direction, on an overground rail system stretching over 320 miles, running at high speed (theoretical uppper speed limit of 250mph), with the London Underground, is laughable. I could not find any references to your claim in the official documents.
The project was never about speed, but rather about increasing capacity. It was packaged and sold to the voters as shaving off some time to get from A to B, largely due to politrickery. The aim is to free up commuter, regional and freight capacity on existing lines ─ which will also have an overall impact on roads and motorway nework used by freight carriers.
https://assets.publishing.service.gov.uk/government/uploads/...
The point I was trying to make was that a comparison to HS1 without accounting for the train frequency is probably not fair.
Also inflation had to be taken into account when comparing projects that are decades apart.
So without knowing much about the details, it might well be that the costs are plausible to what was proposed.