I'm pretty sure that parts of the ECML (not sure which parts, maybe just Stoke Bank where the speed records have been set) were 140mph capable except for the signaling. I think there's even a 140mph linespeed board in the National Railway Museum.I doubt you'll ever see 140mph running on the ECML as long as the eastern arm of HS2 is Government policy. With the regular non stop service to Newcastle from 2019 fastest journey times to Newcastle will fall to being only 15 minutes slower than the quoted HS2 journey time. Remember also that increasing speed disparities between services actually reduces capacity.
Personally I'd like to see linespeeds of 140mph between York and Northallerton, and preferably Darlington if you could remove some of the slow services north of Northallerton.
But I'm not hopeful because as I understand it, it requires significant infrastructure works and not just in cab signalling