No idea as to why retailers outside the EU would be discouraged from trading with anyone in the UK? The rules regarding the way they trade are exactly as they were prior to brexit.
Unexpected customs charges, admin fees and VAT bills are causing chaos for customers ordering from shops based in the EU while some firms have ceased shipping until the problems are fixed.
www.thisismoney.co.uk
You can also find more info on the taxes and duties you may be expected to pay if goods are porchased from retailers outside of the UK here:
VAT, duty and customs declarations for goods received by post or courier - paying, collecting your goods, getting a refund and documents.
www.gov.uk
AS said, these rules have always been in place and are not new.
You'd ordinarilly pay any tax or duty owing to the courier who delivers the goods to your doorstep andf they themn send it to HMRC. It is basically what gave rise to the increase in courier scams that have been doing the rounds over the past 18 months. Scammers have been randamly texting people suggesting they owe money to get goods they'd ordered delivered. The fact is that a courier would ask for a payment on your doorstep and wont be texting you for it.