Wallace, I think you’ve helped me close the argument.
The way I see it, there are two kinds of roads:
Narrow roads where it is impossible for a car to get past two abreast. In these situations just ride single file when a car approaches (from rear or front) or pull into passing place if it’s a single track. This is something I’ve always done and is basically common sense.
Normal road (like most A roads IME) where it is easily possible for a car to overtake cyclists riding two abreast and also possible for two cars and a cyclist right at the side to pass. In these situations where it is quiet, two abreast is fine because there’s plenty of room for overtaking when it’s safe and the car will never overtake when a car is coming the other way (due to two abrest). However, as common courtesy in busy periods as wallace pointed out, one should ride single file out from the gutter to show we’re not being dicks but that we don’t want them to over take then cars are coming the other way (surely this is fair).
Further to this, I believe the policeman was trying to point out that riding two abreast round corners is silly because what happens if a car is coming the other way on our side of the road, you’re better prepared in single file.
I hope I’ve not come across as arrogant / inconsiderate because I certainly wouldn’t think of myself this way.