War accelerated British decolonisation without causing it

Did Germany lose the Second World War but bring the British Empire down with it?

No; the war accelerated a much older transition, and the speed of later decolonisation exposed severe institutional weaknesses rather than creating the process from nothing.

Germany did not simply bring down the British Empire in the Second World War. The transition toward a Commonwealth framework had roots in planning from the late nineteenth century and had gathered pace by the interwar years. The war and later American pressure accelerated that process and made parts of it more abrupt. The institutional argument is that colonial governments typically developed armies and police forces more thoroughly than representative democratic systems. When imperial authority disappeared quickly, those coercive institutions could remain the strongest organized bodies in a new state and seize political power. Later instability in parts of Africa is presented as one consequence for which colonial powers must bear responsibility, although the causal chain is an argument rather than an inevitability.