If single phase railway substations were to be replaced with true 3 phase ones, that could easily increase to 3% of national demand, which is quite a significant increase.The power used by the railway is a tiny fraction of national demand. (I think it's about 1%?) If the National Grid is that short of capacity, then keeping the railway going is the least of their worries.
But looking to the South-West of Manchester as a whole, there is going to be a huge increase in railway electric loads in the next decade or so, with HS2, NPR and CLC all (seemingly) committed to go ahead. So they are going to have to commission one or more major feeder stations in the area. I trust the various development teams are talking to each other!
Luckily there are three 400kV lines (6 circuits) connected to Carrington and Daines sub-stations, so there should be plenty of local grid capacity to tap into. (For details see https://openinframap.org/#13.58/53.42902/-2.38644)
Regional demand south west of Manchester, going from only the Diesel CLC line to an electrified CLC line, HS2 and NPR, will likely represent an even larger increase than that, but since these are all long term projects, they must be making adequate provisions to procure traction electricity when the time comes.