The reason there are no active volcanoes in the United Kingdom is that the U.K. lies far from the edge of any tectonic plates. Typically, most volcanoes occur near plate boundaries where impacts allow magma to well up from below the Earth's crust.
While the United Kingdom does not have any active volcanoes as of 2014, it was home to volcanic activity in the past. The most recent volcano in what is now the U.K. erupted around 55 million years ago on the Ardnamurchan Peninsula in Scotland. Prior to that, the most recent volcanic activity there occurred during the Carboniferous Period.