Nowhere is it stated that passing vehicles on their nearside is actually illegal and this has been tested in court. There is no specific law against it. You can quote “rules” as much as you like. Traffic police can express “opinions” that a driver was driving "dangerously" if he moves from a right-hand lane to a left-hand lane to pass another vehicle and a magistrate might accept this, but it's not illegal to pass on the nearside when staying in lane, and in many circumstances it makes far more sense and is far safer than attempting to pass a slower moving vehicle on the offside. What could be more stupid on a five lane motorway than moving from lane one to lane five, then back to lane one to pass a dawdler in lane four?