The speed issue isn't anywhere near as simple as that. High speed lines in mainland Europe are pretty much standardised at 300 - 320 km/h. Even the increase from 320 km/h to 360 km/h (the initial operating speed for HS2) involves substantial changes to standard rolling stock designs to accommodate 20% higher power and 20% more kinetic energy to be absorbed in crashworthiness. Similarly it involves higher powered substations (or more of them), more robust OLE and so on. There is absolutely no justification for the increase in speed - it's a piece of hubris by politicians at public expense. The capacity issue which HS2 is supposed to solve could have been perfectly well satisfied at 250 km/h, roughly halving the energy consumption per seat kilometre for a start.
Isn’t that mostly due to their lines having been built decades ago when technology wasn’t as advanced? Alstom’s latest TGV has a 350kph top speed, Siemens Velaro Novo has been run up to 360kph. If we’re starting from effectively a blank slate now, why not go with what’s possible rather than constraining ourselves? The marketing claims of the manufacturers are that their new models are 20% more efficient than the old, so negating the increased power demand of the higher speed.
As noted by other commentators, if we get the speed up and hence the travel time down, we can make more of a dent in air travel which is a much worse system than an electric train.