It was a combination of multiple factors that forced the British to leave. Gandhi was a catalyst, a powerful catalyst even who rallied the masses behind him. He had incredible charisma and support from the people of India.

You must understand, India is vast and has tremendous diversity and before Gandhi, never had one person united the whole nation (even here, the extreme South of India or the North East were pretty much unmoved by his exhortations) behind him.

If you look at the non Congress lead, violent resistance movements, they were scattered, non unified and had no one or two rallying points or even a single uniting party. I use these as an example to point out how without Gandhi, the freedom movements might just have been a bunch of disparate movements without much cohesion.

Yes, a few other leaders, namely Patel and after independence, Nehru had some pan India draw, but none could touch Gandhi in this.

Why am I speaking so much about the unifying power of Gandhi? It is important because without this unifying power, the British powers that be might have played their divide and rule policy (whatif? territory) and we could have had unknown consequences.

Another huge aspect, a spectre looming over the British was Subash Chandra Bose and his INA. Atlee himself is said to have recognised the power behind the INA (though version I was easily put down by the Allies) and the 100's of thousands of Indian soldiers, experienced soldiers being demobbed did nothing to remove this fear.

While we will never know what ultimately drove the British to partition and exit India, it was largely a combination of these two factors.