Shellfishes are banned from the menu for royals.
This food rule categorizes more into the list of practical ones, and its importance multiplies, especially when they are traveling. So yes, this isn’t the rule dictated by the Queen, but it is to keep the Royal away from the food poisoning carried by the shellfish when they are away from home.
After all, the Royals are on their diplomatic trips to fulfill their duties which may be compromised if they eat shellfish and catch food poisoning and what can be worse than that. They are also supposed to avoid tap water, rarely cooked meats, or any other food or consumables that may be a cause of foodborne illnesses. So it is better to avoid the shellfish rather than stick their heads in the toilet on their trips abroad.