Hmmmm thinking out loud.....There are a few still in the original Northwestern stars livery....We can discount the 3rd rail gubbins (Southern, Southeastern, Thameslink, Silverlink), can discount VWC, GNER, FCC, C2C & LM cos electric. MML, GC & Chiltern & HT only do intercity. You have said that it is not Central, FGW, Wessex, SWT, FNW, Northern, ATW, ATN or EMT. Can rule out FSR.
Thus I now guess it is TPE (pre 185 introduction) or one...... or at a pedantic push Northern Spirit or Northwestern.....