Under fixed exchange rates, if Britain becomes more productive relative to the United States, what foreign exchange

Question:

Under fixed exchange rates, if Britain becomes more productive relative to the United States, what foreign exchange intervention is necessary to maintain the fixed between dollars and pounds? Which country undertakes this intervention?

Exchange Rate
The value of one currency for the purpose of conversion to another. Exchange Rate means on any day, for purposes of determining the Dollar Equivalent of any currency other than Dollars, the rate at which such currency may be exchanged into Dollars...