The Currency Realised Gain/Loss report lists the currency gains and losses that have been posted automatically through sales and purchases during the month for foreign-currency transactions.
You originally made a sale of £100 that at the time was worth $250. When you received payment, it was worth only $230. Therefore, the $20 difference is a realised currency loss, and will be posted to the British pound receivables exchange account and to the Currency Gain/Loss account.
At the end of the month, you can look at the value of your foreign accounts and use the Currency Calculator (from the Help menu at the top of the screen) to calculate their true values in local currency at that time.
If you looked at your balance sheet, you would see a zero value for the British bank account, but $20 remaining in the British bank exchange account. You need to post the $20 from the exchange account to your Currency Gain/Loss account.