Why changing notes from one denomination to another might aid money laundering is hard to figure...It’s a money laundering prevention measure implemented by the treasury according to our local post office manager.
I know that you can no longer pay into an account without a paying in slip / bank card for this reason. Years ago I did jury service and the prosecution detailed the defendant receiving anonymous payments by giving people a bank account number and sort code. Most paying in slips had either no names or were illegible.