icao
ICAO documents have no force in law. They are recommendations based on an idealistic model of the world and ICAO would be the first to admit that local adjustments are made by the lawful authorities where it is better to do so. As noted above you must follow the aip and the majority of countries do nowadays post the integrated aip as a download on the web The UK, France, Spain and Portugal are shining examples. Some countries, eg Austria, ask a fee.
Some countries such as Italy and the UK are notably "long and thin"; some countries such as France are more "square" but have say a predominant traffic flow E-W. Some passages such as the EURSAM corridor have one direction going in an airfield-rich environment with more advanced navigation aids than you can shake a stick at and the other... er, not. Some passages have the jetstream, some have the ITCZ. Thus there are many occasions where geography and meteorolgy are in conflict with the ICAO world model and commonsense must prevail.
A good example for you to follow once you have all the necessary iaips downloaded for the UK, France, Spain, Portugal and ASECNA is to plan a flight from Aberdeen through Faro, Casablanca, BULIS to Conakry. You will basically be on UP600 and UM322 for most of the way. If you can, dial up Jeppesen weather text. At the portion where UP600 becomes bilateral from unilateral and then becomes UR72 at the FIR/UIR boundary south of Faro to become RVSM of the opposite hand at BULIS it should begin to make sense. Air Law and Navigation orders are mandatory, you must follow the AIP or you'll find yourself in close formation with the military metal.
There are still some real lulus, notably where UG853 not only crosses the equator but also changes hemispherical direction. "You'all be care, yah hear now?" is the rule then and if you don't like what you see coming towards you, just shut your eyes, go right hand down a bit, count to ten and then resume navigation.
Best regards
the "E"