The 8 units due to head south are the same 8 x 158s which came up from England and Wales about 10 year ago, with the tatty interiors.
I was merely trying to eliminate 3 of these I saw at Perth at 5am this morning which were diagrammed to run services today, one of which I'm sure was 870 which I caught to Queen Street (although I can't be certain), which was rumored to have been one of the 2 heading south today. The other 2 units at Perth were '786, and the one which has the saltire seat coverings (which I can't mind the number of), which rules them out of being the ones transferred to Northern.
So if I'm right in saying that it was 870 that worked the service this morning, that means that 2 of the following units would've headed south;
782, 789, 867, 868, 869, 871 [*of which one of these was the unidentified one I noticed this morning].