It's a question of place of supply... You can travel to Germany and buy in a German store and pay 19% VAT (which I believe you can get refunded on exit at an airport - never tried). Then you bring it to the UK and declare it to customs who will charge you import duty. However, if the German company sells to you and ownership of the goods transfers when you are in the UK, it is subject to UK VAT. Now it's much clearer that the place of supply is the UK and not Germany.
I don't know whether you are talking about now or when we were within the EU VAT scheme. That's closer to the current situation, although as we have an FTA of sorts there is no duty payable to customs. It's got no resemblance to how EU VAT worked here and continues to work there.