I often buy goods from UK, including Niche Zero, Londinium and Ikawa.
The answer is very simple. Niche is charging extra 20% for non UK customers (not VAT, the profit stays with them). They have right to do it. Customers have the right not to buy from them because of the extra cost. Is it fair or not, answers depends on where you live ;).
Ikawa does opposite, they reduce the price by 20%. Londinium at some point had lower price for UK customers if I remember correctly (but it was less than 20%). But Londinium do not charge VAT for non-UK users.
Inside UE, things are easier, as in most cases, as end consumer you pay the VAT when buying using the origin country VAT rate.
In US (Kafatek) by default all the prices are excluding VAT, as each state can have different rates.
In all cases (except the EU inside trade), I had to pay VAT at my home country.
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Export from the UK: When exporting goods from the UK to the EU, UK VAT is no longer applicable. Businesses in the UK should zero-rate VAT on the exported goods, provided they maintain proper evidence of the export.