Because it became such a mess I simply gave up.
For example, I used to buy Elemis brand products. They still sell in Euros but here's what it says on their website: "Please note, orders that are shipping outside of the UK will not be charged UK sales tax but maybe subject to customs charges and tax that will need to be paid directly. This affects orders placed both in £ and in Euro. We advise you to please check requirements with the local authority before placing your order." (https://eu.elemis.com/delivery)
Ordering from the UK is the same as ordering from the US or China. Which is ridiculous given the physical proximity. Great job poking yourself in the eye, Britain!
And, worse, there are businesses that have figured out the customs/tax situation and will ship directly to Europe with no problem, but now I'm gun-shy and figure it'll be a mess so I go look elsewhere.