I had the same argument with two mates last weekend who just didn't get it either.
Now admittedly I *would* tend to drop back into single file because a lot of drivers seem to be a bit inept regarding overtaking, but the reality is it's no harder to past two abreast than it is two in single file. Are you scared to get the car over the other side of the white line ? - how you would pass a tractor or any other slower moving vehicle ? If so, get your head round why you're having difficulty - trying to overtake in the wrong place, not looking far ahead or not using visual clues (eg closer central dashed lines, direction signs, junctions ahead) to identify a safe/unsafe overtake points?
It's not unusual for motorcyclists to do some sort of advanced tuition (IAM, ROSPA, etc) - as a car driver you *can* do likewise if you're not confident.
Bear in mind the highway code's kinda done the work for you, in that 'overtake a cyclist as you would a small car' or whatever the exact wording is actually directs you to do the right thing...
If you're in a group of any size it's far better to form a wider, shorter group - whereas if you string out, the person overtaking has straddle/be across the other side of the line for a far longer distance, and is more likely to have trouble finding a good overtake spot.
Even if you've moved over the line, you're not actually commited to the overtake until you're starting to pass what you want to overtake, if you spot something to change your mind - you *do* have the option of moving back in lane, relative speeds permitting.