Why did the British Empire decline?

When thinking about possible theories for the end of the British Empire usually several different reasons come into our mind. On the one hand, there is the economic reason which refers to the time after the Second World War when Britain had lost a huge amount of money and the maintenance of the pound sterling as the main currency was threatened. On the other hand, it could also be possible that the end of this well-known powerful Empire occured in consequence of international reasons. After WW2 Great Britain had to face the bitter truth. They had lost their denomination as "the only real superpower". In the aftermath of the war other nations such as Russia and the USA had become stronger and more influential which made Great Britain suffer a setback.
Of course the above-mentioned theories are mere speculations as none of us is able to establish a general theory which would explain why the British Empire eventually ceased to exist.
However, there is one theory which seems to be the most logical one in terms of the decline of Britains influence. This theory focuses on the end of empire due to colonial nationalism. I suppose that most inhabitants of the colonies already felt oppressed in their countries long long long time before the beginning of the Second World War. They wanted to keep their own identity and did not want to assimilate. They were proud of their own culture and traditions as well as their native language and did not feel the urge to leave their "old life" behind. I can completely relate to their attitude towards the British people. No one wants to be forced to adapt a different and totally new lifestyle. However, it took years until the inhabitants of the colonies finally worked up the courage to refuse to accept the constant oppression they experienced by Great Britain... I bet you have all heard of Ghandi before, haven't you? So yeah, I won't go into too much detail but as you all know Ghandi helped India to gain back their independence in 1947. For sure this was one of the initiators for the other colonies to start fighting against the firm British hold too! And that turned out to be pretty successful as we can see today!