The movement of people to cities in great britain was due to

a population explosion due to better farming methods.

Explanation

The creation of better farming methods makes the number of job opportunities and food availability become located in the cities.

Knowing this, a large portion of Britain population decided to move to the cities in order to get a shot at a better standard of living. Ironically, since most people think that way, it instead created a population explosion that resulted in a large number of unemployment and poverty in the cities.