Though they had wars with Scotland and this and that, they never really had an empire, it was really a united kingdom so to speak. Empire would be what Denmark did to Greenland and Britain did to the Americas. It's difficult to say for sure, but it's that never the less.
Kingdom and empire are greatly different. Kingdom is the direct rule of one man over a small country, while empire is rule by bureaucracy over many largely autonomous entries. Many of modern "kingdoms" in Europe are basically republics, the monarch has so little power. Which is freer largely depends on the period of history.
It's a question of whether or not a given kingdom seeks to expand itself by annexing other countries and manages to do so. Medieval Britain absolutely did both those things, therefore, it was an empire. The line between "empire" and "kingdom" is clear as crystal. Thanks for posting this opinion.