For businesses operating outside the UK, you would set up a Customer GST VAT scheme in order to customise your Sales Tax rates.
Navigate to Settings > Organisation > Details.
Click on the Tax link in the box to the left.
Click on the Manage VAT schemes link.
Select the Custom GST Scheme and fill in the rest of the details of the VAT scheme. Click the Add button to complete.
Now that the custom VAT scheme has been added, custom VAT rates can be included by clicking the Customisable link in the VAT rate column.
This will list all the Sales Tax Rates where you are also able to create new rates by clicking on the Create Sales Tax button.
Give the sales tax a name, a rate as a percentage and tick whether it should appear in sales and purchases lists and then hit the Create New Rate button.