It's quite simple really - in some (most?) companies PF does the walk round, except if it's raining, then the FO does it.
Incidentally, just done my usual MPL - LGW commute with EZY. Watched (presumably) PF do the walk round out of the window - he carefully examined the underside of the engine - looked to be about 18 inches (about 45cm) from the ground - he didn't need to lie on his back!