Why is the supply curve in the foreign-currency exchange market vertical? In the book 'Economics' by Mankiw and Taylor the demand and supply curves on the foreign-currency exchange market are shown as follows:

The supply equals the Net Capital Outflow (NCO). I would have assumed that the exchange rate influences NCO in the same way it influences imports and exports, for example a higher exchange rate makes it more profitable to invest abroad than domestically, leading to an increased NCO. However, the supply isn't influenced at all by the exchange rate according to Mankiw and Taylor and that's what I don't understand.
Because the supply of pounds in the model is controlled and fixed by the central bank. The central bank does not have to change (increase) the supply of pounds when the exchange rate changes. So the supply of pounds will be just a flat vertical line because no matter what the exchange rate is the supply of pounds will not respond to exchange rate as it is given by the central bank's monetary policy.