I have asked this very question directly to the CAA and was told that it only applies to FL100 - as it was getting confused with FL110 and HEADING One Zero Zero (which is never to be called Heading Wun Hundred).
This level was suggested to be "thick with traffic" - climbing out of and descending into airfields.
The trafic at FL200 is normally more spread out and in an intermediate cruise, FL300 even less of a problem.