Hello there, danielgeorge1984.
Welcome to the Community. I'm here to assist you with your concern about creating a tax code for the BigCommerce integration.
You can activate the tax code since applications require this upon the setup. After doing so, you can then default the transactions to NO VAT.
Please note you can turn on VAT without a number or even you're not registered. If you don't want to turn on the feature, I recommend seeking additional assistance from the BigCommerce support.
One of them will be able to assist you further with the integration with QuickBooks. I'm adding their support link for you to reach them (click CONTACT in the upper-right corner):
https://support.bigcommerce.com/s/.
Let me know how the integration goes by posting a response below. I'm always around to help you out.