All that's needed is one platform, cordoned off for about 2 hours. Glasgow has a suitable one - Leeds, Liverpool and Newcastle too. Manchester would be trickier as the best would be platform 1.
13 is probably the next most isolated in a way, but is too short.
I think that Transport Scotland, with NR/DfT, could start on the top bit of HS2 - which in my mind would be a line similar to what we have today, from Glasgow and Edinburgh respectively, meeting and then joining the WCML.
The problem this would have would be pushing Edinburgh trains down the WCML before the HS2 is ready, potentially. To keep it Easct Coast (eventually we should have both) - a Morpeth bypass as part of a future high speed line, would give a lot of benefits to a lot of trains.