There will be a difference in exchange rate between the post office and the bank. It is said that the exchange rate of the post office is slightly better (the difference is not very big! ), you can ask on the same day and compare on the same day. If you want to change it, you need to fill out an order form. Generally, you can't get it on the same day, so you need to get it the next day.
As for the exchange rate, please note that there is a gap between the buying price and the selling price! If you buy pounds from China at the price of 10.98, and then sell RMB, then the selling exchange rate is not 10.98, but about 12, so there will be exchange losses.