For the purposes of this application, Crewe/Weaver Junction area and bits of the north.
I believe the Avanti additional Liverpool services also have this issue - and that is why there are proposals to use bi-modes as a temporary position.
The line is electrified, but because of woeful infrastructure and years of delay/HS2 uncertainty, there is not enough electricity being supplied to support additional electric services, however bizarre that sounds. However, batteries etc are increasingly viable.
It also doesn't factor in that a lot of planned electric freight paths actually run as diesel - so there is probably more electricity available, but because freight operators have the right to the electric, you couldn't rely on that.