From various shipping information sites, namely:
Starting January 1, 2021, shipping to the UK is becoming more complex due to Brexit. If you’re an online retailer shipping products from the U.S. to the United Kingdom (UK), you will need to change your fulfillment process starting January 1, 2021. The UK has left the European Union trading...
blog.stamps.com
E-commerce merchants in the US looking to expand globally should first consider shipping to the UK. We explain why that is and how to do it.
goshippo.com
pe.usps.com
We're going off topic here, but I'll try and reply, it is interesting information, but both articles deal with Business sellers, who are presumably selling high volumes of their products to the UK.
Individuals, or companies that seller much smaller quantities, don't need to go through all that hassle, they can, and do, just fill in a Customs Declaration form, and post, as long as you've told the buyer they are responsible for import taxes etc. your good.
If you sell through someone like eBay and use their international postal system, they do it for you, the seller then only receives the actual item price, nothing else, so do not need to register with HMRC as eBay are the registered importer and they pay HMRC any taxes due.
However, I, as a private individual buying say a Model Locomotive, for myself,
I do not need to pay import tax. any any item valued at under £135. But the Taxman wants his share, so they have a sneaky way of getting it.
If say I buy from you a Locomotive, value £110, postage and insurance, is £35, total £145, I have to pay Import Tax and VAT, if the postage was £19 then only VAT is payable.
Also A bug bear I have is that Sellers in the US don't correctly fill in the Declaration form, there are two important boxes on the form,
1) Description.
2) Category or Tariff Code.
1) should read "TOY" invariably they write "Merchandise" however merchandise means I am importing the item for resale not private use.
2) is usually left blank, (Tariff/ Category Code for a Model Locomotive is 950300) so HMRC don't know exactly what's in it, and the correct Import tax (if any) to apply, so they error on the side of caution and assume I'm a business (because Description box say's "Merchandise") which then attracts a higher % of import tax.
That means after I've paid the Import Tax and VAT, and received the item I have to print out the original advert for the item, (usually a screenshot) two pages of documentation, when, where, price paid, postage etc. and a foto's of the actual item, including the packaging and send that off and hopefully get either a full or partial refund of the Import tax.
This is a link to the UK Government Tax and Customs website for goods sent from overseas, if your interested.
VAT, duty and customs declarations for goods received by post or courier - paying, collecting your goods, getting a refund and documents.
www.gov.uk