Everything about UK rail is custom (apart from the gauge). Apparently it's one of the (many) reasons HS2 is such a mess.
They were trying to run trains faster than typical continental high speed lines, which meant custom design work that needs loads of additional testing and certification. Rather than just use the Spanish or French high speed designs.
Max line speed of HS2 is 360km/h, with provision for 400 in some sections in future. This is entirely in line with many other modern HS lines. China’s been running regular 360km/h services for years.
This is a project with a 200+ year shelf life. Designing to 300 or less would have been short sighted, and many of the changes to accommodate such high speeds actually reduce costs in the long term (slab track, headroom to catch up delayed services, ability for one trainset to operate more services per day etc).
The cost overruns of HS2 are primarily from plain old poor project management, complex planning law and constant political meddling, not engineering decisions.
China has run at 360kmh before, but last I checked they mostly run slower.
Air resistance is a killer as you go faster. So for trains 300 is usually about the best compromise between energy use and speed. If you want to go faster a jet at altitude is going to be much faster at a better fuel efficiency. High speeds make sense for long distances.
They were trying to run trains faster than typical continental high speed lines, which meant custom design work that needs loads of additional testing and certification. Rather than just use the Spanish or French high speed designs.