Hi gorgeousgames
If you are a Non Established Taxable Business and make taxable supplies in the UK then there is no Registration threshold and you would have needed to register after making your first sale.
Please nsee the guidance below:
Non-established-taxable-persons (NETPs) — basic information
In order to account for this VAT retrospectively you would need to account for these on your first VAT return
If the sales you have made are of goods of a value below £135 then it is the responsibility of the marketplace to account for the VAT on your behalf and so if you do not make supplies of goods over £135 then there will be no requirement for you to register for VAT in the UK at all.
We would recommend speaking to the marketplace about this.
Please see the guidance:
VAT and overseas goods sold directly to customers in the UK
Thank you.