Does the word "humble" used here is to express sarcasm?
No, not in this situation. John Andrew's tone is not mocking, and if he were sarcastic, he wouldn't have said "idiotically." If he was trying to be sarcastic, he would have used a mocking tone and said something to the effect of "My country made a brilliant decision to leave the European Union." In this situation, he is expressing his opinion and is trying to be informative, not sarcastic.
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