How far do economic factors explain the expansion and dismantlling of Britain's empire?
Economic factors were very important in the expansion of the British empire but rather less important in the empire's decline.
In the period of expansion, economic factors were very important. It was economics that drove the growing power that Britain held over India. It was economics that made Britain push into China. It was economics (the need to protect trade routes to India) that first got Britain involved in Northern Africa. There were other considerations, such as military power and imperial prestige, but economics was the driving force.
The dismantling of the empire, however, was less driven by economics. Instead, it was driven (as in the granting of Indian independence) by political forces within the empire. And it was driven, as in the case of the decolonization of Africa, by pressures from the United States that had to do with the geopolitics of the Cold War.