SS,
No, ICAO is not 'the law'.
The civil ATC 'law' in the UK is provided by CAP393, and enshrined as procedure by the CAA in CAP493 and each ATC unit's MATS Part 2 (or Pt.2 for Geffen!).
The UK does some things that are
not in any ICAO Manual/SARPs/Procedures.
The UK does not require some things that
are in ICAO.
Hence why differences are filed with ICAO by states all around the world.
But as you have now discovered,
even ICAO allows us to do it.
