Just to clarify something, I am totally in favour of the self-determination of the Kelpers (or any other people for what matter). My point is different, regarding colonies, the ICJ can only provide non binding ruling and advisory on this type of issues. It does not have the mandate nor the jurisdiction to impose a solution. Countries may choose to put themselves under jurisdiction the same way that countries may accept or not the non binding resolutions of the UN. Argentine can choose to not attend the ICJ the same way the British government ignores the UN decolonization resolution to seat down and negotiate with Argentina.
Again, the ICJ per se does NOT have jurisdiction or mandate on this issue. The mandates come from the countries. The key words are once again mandate and jurisdiction.