This may have been asked several times, but why couldn't the Eastern Leg be built first of the two rather than the main stretch to the north west?
There have always been doubts over the route of the eastern leg, notably about how to serve Sheffield (the current plan takes it well to the east, with a loop through the city on the classic line).
The western leg has (so far) not suffered the same uncertainties, but it still has to get through the local opposition in leafy Cheshire.
The proposed NPR/HS2 connections in Cheshire (towards Liverpool) also seem to complement the overall scheme, but the wider NPR route is still unknown.
What happens with NPR at the Piccadilly terminal is still unresolved, however (through station underground or surface reversal).
HS2's northern rolling stock depot is also supposed to be on the western leg, north of Crewe in the angle between HS2 and the WCML.
Simon Jenkins is right in the sense that the planning issues on the eastern leg may well sink it, at least until NPR is built (which also has huge planning and technical challenges, but has wider political support).
Parliamentary and government committees are also often hostile to mega-projects, as they are rarely efficiently delivered (cf Crossrail, aircraft carriers etc).
They also suck up vast taxpayer funds which are denied to all the other priorities.