For orders added in the back office, the tax code rate (sum of tax components) will be used to calculate the tax amount to apply to each order row.
For orders downloaded from a sales channel, the tax amount is determined by the channel.
For downloaded sales orders, Brightpearl assigns a tax code to the order row based on the following rules. Tax codes can also be manually updated on order rows prior to invoicing the sale.
If you are a UK based seller and make a sale to a UK based customer, the tax code is usually driven by the product. So to ensure this is the case, do not assign any tax code to your United Kingdom tax zone, but ensure your products are assigned the relevant tax code.
If you are a UK based seller and make a sale to a customer outside of the UK, then the tax code will usually be driven by the location of the customer. So to ensure this is the case, assign the relevant tax code to your tax zones at Settings > Tax > Tax zones.