They’ve already built the most expensive bit; all the tunnels under Home Counties Tory constituencies. The line to Birmingham has cost ~£100bn. To finish it to Manchester would cost another ~£30bn for about the same distance again. They should at least finish it to Crewe where it can actually start to relieve traffic from the West Coast Mainline which is full; you can hardly ever find tickets less than £150 return and trains are often packed. Compared to TGV UK trains are double the time double the price and half the legroom and too much vibration to easily be able to select a paragraph of text with a trackpad. Transport emissions are significant part of UK emissions and replacing 150gCO2/km journeys with 9gCO2/km would help to cut into these massively.