Hi,
If you are an overseas business and you are making sales of any value to a customer in the UK via a marketplace then it should be the responsibility of the marketplace to account for the VAT. You can see guidance below:
Goods in the UK at the point of sale — sold by an overseas seller through an online marketplace
There are 2 sales taking place in this scenario. The UK supplier is making a supply to you and you are then making a sale via the marketplace to the end customer. As per the guidance above, when you sell the goods to the end customer you are making a deemed supply zero rated supply to the marketplace. This will allow you to register for VAT and recover the VAT which you will be paying to the seller of the goods to you in the UK.
Thank you.