It would be a dreadful shock to British morale. There would be a crackdown on all "suspicious" foreigners, especially radicals. This in spite of the fact that the attempt occurred in Belgium. Quite possibly it would mark an end to foreign travel by the Royal Family.
Would Belgium be held responsible? I don't think so, especially as Sipido was 15 years old.
George V succeeds to the throne when Victoria dies - which might be earlier, due to shock and grief at the death of her son. He's 36 instead of 45.