They didn't exactly choose, their hand was forced by the American Revolution. Before the American Revolution, they very much considered North America to be the most important part of their overseas empire. Their holdings in India consisted of a few trading posts and they had just recently acquired Bengal. After the American Revolution they had lost the core of their North American holdings, so their attention shifted to India. They rapidly expanded their territory in India until they controlled nearly the entire subcontinent, and to secure their trade routes to India they also conquered South Africa.

Thus the British Empire is divided into two eras: the [First British Empire](https://en.wikipedia.org/wiki/British_Empire#%22First%22_British_Empire_(1707%E2%80%931783)), focused on North America, and the [Second British Empire](https://en.wikipedia.org/wiki/British_Empire#Rise_of_the_%22Second%22_British_Empire_(1783%E2%80%931815)), focused on India.