Since January 2021, the UK is no longer part of the EU customs union. US sellers shipping to the UK must treat it as a separate customs territory from the EU — VAT and customs duty are assessed independently.
UK VAT is 20% on most goods. US sellers shipping to UK consumers must either register for UK VAT and collect it at checkout, or use a marketplace that handles VAT on their behalf (Amazon UK handles this for FBA sellers).
The low-value threshold for UK import VAT was removed — all goods imported for sale in the UK are subject to VAT regardless of value. This affects small-parcel B2C shipping directly to UK consumers.
For FBA on Amazon UK, the import VAT is handled as part of the FBA program. Amazon collects VAT from customers and remits it. The complexity for sellers is primarily in the shipment of inventory to UK FCs, which requires a commercial invoice and UK import declaration.