That's the main complication - all there is is (as things stand) is a single crossover at Parbold and the pointwork at Southport, which judging by NW6009 sequences/pages 4 to 7 is not exactly conducive to a run-around itself, either. If you are going for a pad, though, you can leave the wagons in place or have the loco propel back to Wigan Wallgate junction where a through siding is available, or even the junction.
If there is a long enough time slot, a propelling move is possible. I don't know what the score would be like for signalling such a manoeuvre, however, as "setting back" generally requires direct authorisation from the signaller.