As highlighted in the excerpts above drivers should be in the left hand lane. If they are over taking they should pull in as safe to do so. If the driver doesn't complete the manoeuvre they aren't using the road in accordance with the highway code, so completing the already started overtaking procedure wouldn't be "unnecessarily changing lanes".
What if it wasn't safe to do so at that point?
And don't prattle on about he shouldn't be overtaking then.
And the prat in the OP shouldn't have been straddling the central line as he wasn't performing a manoeuvre in accordance with the highway code.
As for the other route (past Amberley, Storrington, Steyining), I normally use that route when travelling from Shoreham to Portsmouth if it's at a time when there are hold ups at Worthing and Arundel. It's longer mileage wise, but you keep moving.