Hi Mark Harris,
If you are a UK established business and selling products in the UK then there is no requirement to register for VAT unless the Registration threshold is breached.
You can however register voluntarily if you wish, please see the guidance below:
3. Working out whether you need to register
Unfortunately we can't produce confirmation to you that you don't require a VAT Registration and so you would need to contact the marketplace regarding alternative evidence you can provide them.
Thank you.