From a foreigner's perspective, the HS2 plan was overly ambiguous. When Japan first built the HSR, they aimed to connect their two largest cities, much like HS2 Phase 1. They then gradually connected to other cities, following a rolling electrification plan to avoid boom and bust cycles.
When HS2 firstly proposed, the UK's railway sector was very limited from both an engineering and research perspective （and still limited for now）. Can you imagine that both China and Japan had multiple railway universities training engineers and researchers in railway-related fields? An optimal solution could have been to build things gradually, build the team, and expand capacity in a rolling approach. In fact, in China, they first conducted a 100-mile trial HSR from Guangzhou to Shenzhen in 1994-1997, then implemented a flat HSR line of around 300 miles in Northeast China from 1998-2003. Afterward, they initiated their first real HSR project in 2006, which was completed in 2012. However, even in this case, the first HSR project in China was overbudget by 2.2 times.
