Thanks for checking in with us. Let me provide some information concerning VAT codes in QuickBooks Online.
For the correct VAT usage, I would suggest speaking to an accountant or reach out to HMRC to check what VAT boxes should post the US transactions. If you are on the Flat Rate Scheme, you will find that some of the codes will not post in the same way.
When issuing sales or purchases in QuickBooks, you can check these articles to know what VAT rates to select: