London is supposed to represent West African origins( #1 producer Ivory Coast, Ghana ... ) and NY South East Asia. Not sure of the exchange rules, I don't know if someone can take delivery in London , ship it to NY and redeliver it on ICE US. Anyway, the relationship is a loose one. It is mean reverting on the long term but we have been at these "extreme" levels for 2 years now. Originally, it was because of some shitty beans from Cameroon in London warehouses. The ICE pdfs on their products are really informative if you need some guidance.