Hi Charlie shailer,
If the goods are in the UK and you supply goods to UK customers and US customers then both will be considered taxable supplies and will count towards your taxable turnover.
The sales to US customers are zero rated exports but they are still taxable supplies.
If you arrange goods to be sold from outside the UK to a USA warehouse then this will be outside the scope of UK VAT as the goods are never in the UK. You can see guidance here:
Working out whether you need to register
Thank you.