The UK will opt for a mechanism similar to the One Stop Shop (OSS) coming in on mainland Europe from 1 July 2021, but in the case of the UK this will be introduced when transition period for the UK to exit the European Union ends.
From 1st January 2021 onwards, where non UK suppliers send consignments not exceeding £135 in value to English end consumers, no import VAT will be due by the foreign supplier. Instead the supplier will need to be UK VAT registered and account for UK VAT on the sale.
If the sale is made through an online market place (OMP), the OMP will be liable to account for the UK VAT and not the overseas supplier.
Apart some exceptions, when the consignment value exceeds £135, UK import VAT will be due and the foreign supplier will have to register for VAT in the UK to recover this VAT and charge VAT on the sales, unless the customer acts as the importer and pays the import VAT.