In the unofficial market, the pound was trading at 7.38 to the dollar, slightly weaker than Thursday's rate of 7.35, one trader said.
The rate for dollar sales has stayed in a tight range since the beginning of June, after the bank allowed the pound to weaken in May. The central bank offered $40 million on Monday.
The rates at which banks are allowed to trade dollars are determined by the results of central bank sales, giving the bank effective control over official exchange rates.