On the basis that a picture paints a thousand words, I thought a sketch might elucidate the problem, so far as it could be called a problem.
The top drawing shows the make up of the South Wales service using right and first handed coaches with the corridor down the same side throughout the train.
The bottom sketch shows the same train without handed coaches, the corridor switches sides, although one of the thirds could be turned around so that the corridor each side of the restaurant car was on the same side, swapping over halfway along the train.
In his book, Harris suggests that the handed composites ensured that the first class compartments were closest to the restaurant car, presumably so that first class customers didn't have to walk to far to eat!
I know when the GWR stopped building handed stock, but does anyone know when they stopped bothering about marshalling the corridors along the same side, or did this only happen on the major expresses? Anyone know?