> Complete rethink/reboot required.

Or, instead, keep building, so the UK actually gets experience with large scale projects? Establish an anti-corruption body that retrospectively investigates every pound spent on HS2, and places lifetime public-contract bans on contractors found to have acted dishonestly? If the graft is as extreme and obvious as you say, surely this is no hard task.

If the UK has no experience building things, there's only one way to get some, and it's not to stop building for ten years while the government 'rethinks and reboots' (i.e. pays McKinsey for expensive reports exculpating McKinsey for any cost overruns). Ten years during which all the people who were actually involved move on to other roles, often private sector, often overseas. That's how you throw away all the experience accrued during this construction.

Sometimes the perfect is the enemy of the good. In twenty years, when the HS2 is zipping around, bringing down the cost of logistics, making groceries cheaper, lowering house prices as people can live further out, no one will even remember how it was built.