Hi @mcrowne
Unfortunately, there isn’t a way of doing this though the settings, however if you have a Power User Subscription, there is a way of getting it to display on invoices and statements using custom CSS.
If you would like to see this added in the settings, please add your vote to this feature request: Pound sign on estimates and invoices rather then GBP
If you would like to know how to add it using custom CSS, please see the following: Pound sign on estimates and invoices rather then GBP - #4 by Oikake