I understand that you have set up a new Xero organization to keep the accounting records of a Spanish Company. The base currency is GBP.
This is not allowed. A Spanish company must use as functional currency the EUR, which means that all the accounting reports must be drafted in EUR. Moreover having to deal with currency exchange rates for a lot of transaction (as most bills / invoices are likely to be in EUR) seems a nightmare. Finally, if the base currency in Xero is not EUR you will create artificial currency exchange gains / losses which should not take place. If you receive an EUR bill and pay it in EUR, you will not have any currency issue. However, if your base currency in Xero is GBP, then you may have currency exchange gains / losses in the books for the difference in exchange rates EUR-GBP between the bill date and the payment date. This cannot happen.
Finally, be aware that once a Xero organisation has been set up, it’s not possible to change its base currency. So it is highly advisable to chose EUR as base currency from the outset.