Final answer:
The mid-1700s population growth in Britain was due to agricultural advancements leading to better nutrition, the Industrial Revolution causing urban migration, and natural population increase combined with immigration.
Step-by-step explanation:
The population of Britain increased in the mid-1700s due to several key factors tied to economic and social developments. First, agricultural improvements in the previous century, including the introduction of new staple crops like potatoes, led to increased food production, using less labor, and improved nutrition. This allowed the population to grow, creating a workforce for emerging factories. Second, the Industrial Revolution prompted a rural-to-urban migration as people moved to cities for work, contributing to the urban population growth. Lastly, there was natural increase and immigration due to more stable living conditions, which saw the population in the middle colonies thrive. Combined, these factors greatly expanded Britain's urban and colonial populations.