Officially it was :The report of Sir Hermann Bondi on the North Sea flood of 1953 affecting parts of the Thames Estuary and parts of London.
More likely it was the thought of the London underground network having to be pumped out that spurred them into action.
Plus all of the other tunnels under London, both known and secret.
https://en.wikipedia.org/wiki/Thames_Barrier
Wow. Great article!