Because it came after a bit about being considerate to other road users In my mind I had that you were recommended by the Highway Code to ride single file on narrow roads to let cars past. In fact as others have pointed out, you should ride in single file on narrow roads, or busy roads or bends whether or not you are obstructing traffic.
I still however firmly hold my belief that the Highway Code insists on tight fitting clothes and forbids waving to other cyclists.
Edit: ... and doing track stands at traffic lights is compulsory.
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