If you are using total to words in your estimate/invoices you need to add the currency words manually into the translation.
business24 comes with 2 predefined currency names who are already added into the converter. If you added your own currency you need to add this also to total to words converter.
You need to add your currency database name and the currency string who will be outputed inside the invoice/estimate.
Note that the GBP is the same like the name we have setup inside the CRM (showed in the image);