This story is being interpreted a consequence of Brexit, but it's really a consequence of new VAT rules on imports to the UK from last year. The only Brexit link is that EU trade is no longer exempt from these rules as the UK has left the Single Market
1/5 https://www.bbc.com/news/business-55530721

The @BBCNews article correctly explains this:
"VAT is now being collected at the point of sale rather than at the point of importation, a change that HMRC says will ensure that goods from EU and non-EU countries are treated in the same way."
2/5

"This essentially means that overseas retailers sending goods to the UK are expected to register for UK VAT and account for it to HMRC if the sale value is less than €150 (£135)."

... which is a major pain, and probably not worth it unless you are selling large quantities.
3/5

Non-EU firms already discovered this in autumn last year, notably including @WilliamShatner a.k.a. "Captain Kirk", who decided he'd rather stop selling to the UK than deal with the hassle of registering for VAT in another country.
4/5 https://twitter.com/WilliamShatner/status/1316887009198141441?s=19

The consequence of this are that:
A) some goods will be unavailable - until the seller has found a commercially viable work around
B) higher prices - a new market for middlemen who take on the VAT burden as wholesale importers and profit by passing the costs to the consumer.

5/5