P12 was used primarily to house Northern units which sit in Newcastle for long periods, and thus only saw occasional journeys departing from it (the one-a-day Newcastle to Whitehaven direct service being one I recall - a relic of the past now) though I did see the occasional Crosscountry unit using it. Like other's have said, it was disconnected and taken OOU to allow lengthening of P11 to accommodate 802 usage, where previously they were having to take up the through platforms.
From December XC are primarily timetabled for P10, TPE for P11, and a Northern unit will sit in P9 for most of the day.