There’s a tool available from the USD account’s settings page to account for currency gains or losses. Every foreign currency account maintains two balances, the currency balance you see and a hidden GBP balance based on the exchange rates you applied to each payment in or out. Over time this will drift away from the actual GBP balance according to the current exchange rate, the loss/gain tool recalculates the GBP balance and moves the difference to the “currency charges” nominal (which will make it an expense on your P&L).