Hi Ast,
If you are based overseas and you sell an item below a value of £135 to a private individual in the UK then you would need to register for VAT in the UK straight away as there would be no threshold to breach as a non esrablished trader.
Please see the guidance here:
Changes to VAT treatment of overseas goods sold to customers from 1 January 2021
If you sell goods over a value of £135 then your requirement to register will depend on whether you are the importer of the goods.
If you are the importer of the goods then you will be making taxable supplies in the UK and so again you would need to register for VAT immediately.
However, if your customer is the importer of the goods then you will not be making a taxable supply in the UK and so there would be no requirement to register for VAT in the UK.
Please see the guidance here:
Who should register for VAT (VAT Notice 700/1)
Thank you.