Why westward flight takes longer time?
This is simply due to the jet stream. High altitudes fast air current flowing west to east. Therefore if you are flying against them your ground speed is reduced and it takes more time to go from London to New York than the other way around. Jet stream exist in both hemisphere but they pass through middle latitude in Europe USA and Asia wherase they mostly circle Antarctica in the south. Thus making them less important for aviation.

The jet stream is the main reason why the North Atlantic tracks are updated twice a day to ensure they are aligned at best with the winds to reduce flight time and fuel consumption.