Cold-water expert Dr Mark Harper says: “Once you have control of your breath, that is probably long enough.” Remember, rivers and lakes are usually colder than the sea. 11.

Datta, R. L., Napier, D. H., and Newitt, D. M., Tram. Brit. Inst. Chem. Eng., 28, 14 (1950).

Then there’s demographic change to factor in. Since the Victorians built their water network, the UK’s population has grown enormously, from around 40 million in 1898 to an estimated 67.6 ...