This animation shows the changes in position of the areas currently occupied by Britain and Ireland (in red). It starts 450 million years ago and moves towards the present day in steps of 10 million years.
Over this time, sea levels around the world were periodically rising and falling, changing the boundaries of land and sea. This aspect of change over time is not shown in the animation.
The theory of plate tectonics explains why the Earth's continents change their shape and position over time. A detailed explanation of plate tectonics is beyond the scope of this web site.
For those who would like to learn more about plate tectonics, a visit to This Dynamic Earth, a web site of the U.S. Geological Survey, would be a very good place to start.
Also valuable is the BBC's web site Britain's Rocky Past which explores plate tectonics from a British perspective.