OK, that's a lie, they're almost certainly not vampires, but nobody is to serve garlic from the Royal kitchens. They won't even have it in the house (or Palace, for that matter). According to John Higgins, who was a chef at Buckingham Palace, the Queen cannot stand garlic and so refuses to have it anywhere near her.
It may just be down to taste, but there are also likely to be practical reasons for her dislike of it. Garlic is known to make people belch, which would just be unthinkable for a member of the Royal Family to do in public, and of course it lingers on the breath for a long time. It just wouldn't do for the Queen to go out and visit well-wishers smelling like she'd just indulged in takeaway food. Knowing this, we recommend that nobody eats garlic shortly before meeting the Queen, either.