It possibly is fair to say that, after World War One, the rot had set in. For all the Empire grew until 1922, Britain was heavily indebted and also the sacrifices of Anzac soldier at Gallipoli etc had borne a sense of entitlement that they deserved independence. Ireland fell apart during World War One also albeit that had brewed for decades. While decolonialisation really took hold post World War 2, the roots started 30 years prior.