To convert the average rainfall in London from millimeters (mm) to meters (m), you note that 1 meter is equal to 1,000 millimeters.
Given that the average rainfall is 557 mm:
\[ \text{Rainfall in meters} = \frac{557 \text{ mm}}{1000} = 0.557 \text{ m} \]
So the average rainfall in London each year is 0.557 meters.