What was the most important factor of the British Industrial Revolution?
As I understand this question, it is asking what the most important factor is that caused the Industrial Revolution to begin in England rather than somewhere else. If this is the case, I would argue that geography was the most important factor. There were a number of geographical factors that made England particularly suited for early industrialization.
First, transportation in England was relatively easy. England has a large number of rivers that could be navigated by sizable boats and ships. England has many natural ports. In the time before railroads, it was much cheaper and easier to move things by water. Because England was well-suited to water travel, it was easier for the Industrial Revolution to start there.
Second, England had relatively good sources of power. The rivers were part of this as they made it possible to use water power to run machinery. In addition, England has large coal deposits that could be excavated and used to power machinery.
Finally, England had other resources as well. In particular, it had large deposits of iron ore. Iron is a very important metal for industrialization.
All of these geographic factors are generally cited as the reasons why the Industrial Revolution began in England rather than somewhere else.