Good question.
On the Sidcup line, these trains would have been against peak direction, which might have been less of an issue, but can't think of anywhere on the south eastern bit they could have been looped anywhere, unless they went in to one of the yards at Hither Green, but that would have meant the loco would have had to change ends.
What route would they have taken? I'm guessing Midland as far south as Cricklewood, then the Dudding Hill line, Kew Bridge, Clapham Junction, South London Line, Nunhead, Lewisham.
There were south eastern trains via Lewisham and Nunhead at that time, but possibly less than now (from memory, just a few peak direction Dartford - Blackfriars / Holborn Viaduct trains) but would have thought that fitting freight trains through the junction at Lewisham somewhere round 0830 and 1730, even if in the off peak direction, would not be that good an idea.