He could make laws, make war, raise taxes, and a number of other things. Parliament is sovereign over those matters, and only those matters, which it can determine simply by choosing that they should be so. In those cases National sovereignty is not the same thing as the sovereignty of the ruler. However, in the case of the UK National sovereignty is almost identical with Parliamentary sovereignty, so here we shall treat them as the same thing. We need just one more distinction before we can apply our concept of sovereignty.
Sovereignty is different from control.
Parliament might pass a motion condemning some law in Australia. And since the Australians respect the British Parliament this condemnation might make them change their law. But the fact that the British Parliament was able to bring about a change in the Australian law in this case does not mean that Parliament is sovereign over this aspect of Australian law. Again, some Latin American countries are heavily dependent on the coffee industry. Parliament might pass a bill which give favourable treatment to Latin American coffee, thus leading to lower unemployment in those countries.