If staying by the rules, no other heir to the thrown is allowed to fly together.
Being higher to Britain’s royal family comes with a lot of responsibility, and this rule definitely is to put the backup plan in place. In case of worst situations, this is the rule that the royal family members cannot fly in the same plane as another heir. This is because if something goes wrong, Britain may not end up losing all of its heirs to mishaps related to the plane.
However, we saw Prince William not abiding by this rule, at least not always. He prefers traveling with his children, which he also clarified by making a formal announcement that he is never going to fly without his children as he fancies traveling together as a family. It is certainly an exception to the royal rules of flying.