The issue here is the desire to use the "self-contained" Paddington-Abbey Wood section for testing without requiring a working interface between signalling systems. How do you propose to move trains from one or other of the depots to the core, which necessitates movements using at least two different signalling systems, without a working interface between the signalling systems?
There is, of course, no issue at all with moving trains between the eastern and western operating sections not via the core, but that's not relevant here.