"used to be" prior to World War 2. The loaves shrank (and ingredient quality worsened) as part of rationing. After the war better ingredients became available but loaves remained 14oz, which is slightly less than 400g. Conveniently 14oz is not enough under 400g to count as "undersize" in the regulations, so generously filled 14oz tins produced compliant 400g batches when Britain went metric.
I knew of the Bread Act that set it 1lb/2lb, and the Middle Ages origins that worked "backwards" - when the price of the loaf was fixed and the weight of the loaf was varied as the price of wheat changed.
