Do I need an ATaR or a BTI as a UK seller?

The destination of your goods determines this, not your company's location. When you import goods into Great Britain—England, Scotland, or Wales—you apply to HMRC for an Advance Tariff Ruling (ATaR). When you move goods to Northern Ireland or the European Union, the BTI system applies, which is substantively equivalent to the EU's vZTA system. If you sell to both continental Europe and within Great Britain, you need both rulings, because each binding covers only its own legal territory. A single ruling does not automatically cover both trade streams.