For me it's summed up by the £100M tunnel to protect bats. Someone says the nice bats in those nearby woods might not get on with the big scary trains so £100M gets spent to resolve the issue. Scale that kind of thinking up over the whole project including people who don't want HS2 at all using every legal angle imaginable to frustrate it and there's your £66Bn.
There are no adults in the room saying you know what, the value to life and society and the good that could be done with £100M of public money is worth more than the unproven possibility of a bat being injured.
One of the good things and assets of this country is our strong legal system and the comparative accessibility of justice, compared to many other places in the world. But this also gets used by people with an axe to grind to frustrate big public projects.
Be careful when getting irate about aledged "Health & Safety" or "Ecological" oe other money wasting. The facts are often different than they first appear: https://jeffollerton.co.uk/2025/01/31/no-the-hs2-bat-tunnel-...
Central London monthly rent for a 3-bed house is around £3500, but a 1-bed flat rents for £2100. So split the house into two flats and the landlord makes more money.
Hence relaxing zoning restrictions will push prices up.