I've got a copy of the 1972 BR Rule Book.
Rule H 7.4 states that beside the normal train tail lamp, goods trains not completely fitted with automatic brakes must also exhibit the two side-lamps red to the rear and white to the front when travelling on the main line, fast lines or single lines.
But on slow lines, relief lines, goods lines and loops next to main lines or fast lines where trains move in the same direction, the side lamp nearest to those lines must be turned round to exhibit a white light on that side; this applies even if there is a line used in the opposite direction between the two lines (well, that's what the Rule Book says!)
Where the goods line or loop line is next to a slow or relief line with trains moving in the same direction, the two side lights are left showing red to the rear.
On entering a reception siding the sidelights must be removed or obscured after the whole train is in the siding.
This Rule appears to be almost identical to a similar rule in an earlier rule book I have dating from 1933.
Regards,
John Webb