That £19bn in 2013 would be £28bn in 2025 money, add in a 50% cost increase on everything due to "over engineering" and we get to £42bn. Add in trains and contingencies and we get to £50bn.
That's the p50, the p95 was 21.4bn in q2 2011 prices. With rolling stock at p95 and in 2019 prices, the current base, the original budget for phase 1 was £30.5bn. https://assets.publishing.service.g...hment_data/file/286797/financial-case-hs2.pdf
At go ahead in 2020, the budget was revised to £44.5bn in 2019 prices.