Your getting my brain all stirred up.
I started thinking maybe there was a need to test that the 387/2s still worked on AC AFTER a spell working DC. There are only two places I can think of where the switch over can be tested the first is Farringdon-CityThameslink (on the Thameslink core) and the second is North of Sheppard's Bush (on the West London Line). I would have thought the second is less of a disruption if something goes wrong.
But then maybe the 387/2s were being used to train more drivers on 387s - possibly new Thameslink drivers, possibly drivers borrowed from non-Thameslink, possibly even Gatex drivers (as they would not know either of the above routes ?).
Whatever it is would this be the reason why the two 387208 & 387209 were at Tilgate Sidings (Three Bridges).
[/brainstorm]