The earth's lithosphere is cracked into a number of large rigid slabs called plates. Where these plates meet is the plate boundary. The plates move, floating on the soft rock, beneath the lithosphere.
Some plates carry just ocean basin but most have both continent and ocean basins.
Britain is on the Eurasian plate. Most earthquakes, volcanoes and mountain forming takes place at plate boundaries. The fastest plates e.g. Pacific Plates, move at over 10cm a year. The North American and Eurasian plates are among the slowest movers. New York and London are moving apart by between 5-10cm each year.