Thanks for posting in the Community, @figoegebeur-gmai.
QuickBooks Self-Employed (QBSE) features simple invoicing, which helps create professional-style invoices for use in billing your clients.
Currently, the system is designed to use one currency when sending an invoice in which the option to send other currency is unavailable. As a workaround, you can convert the amount of the invoice from Euro to Pounds.
To know more about how the invoice feature works in QBSE, see this article: Create invoices in QuickBooks Self-Employed.
If you have further questions about invoicing, please feel free to reach back out. Have a great day!