US seller with VAT registration in the UK. How do I calculate VAT?

I’m a very small small seller in the USA so I’m trying to do everything by myself to save money. That being said, can someone help me with the VAT in the UK?

I have products in FBA UK, I also ship products directly to buyers in the UK and EU:

1. Do I have to pay VAT for products I shipped from the USA to buyers in the UK and EU? I figure they have to pay import duty so I don’t have to pay VAT again.
2. Do I have to pay VAT for products Amazon shipped to EU buyers from FBA UK?

Thank you very much in advance!