What about Philip and Mary having only surviving daughter? Any sons born from Philip's third marriage would have better rights to Spanish throne than her so even if Don Carlos is childless Anglo-Spanish union is not going to happen. She will be around 10 years old taking the throne, so not married yet-Philip would like to marry her to one of his Habsburg cousins (archduke Charles or one of Maximilian II's sons) before end of regency to keep England in Habsburg sphere?