From your description, the only income appears to be the sale of advertising. Provided that this doesn’t exceed the VAT registration threshold (currently £85k), you won’t have to register for VAT.
However, it may be beneficial for you to register voluntarily. If your customers are VAT registered businesses that are entitled to reclaim the VAT on their expenditure, you could add VAT to your charges, at no cost to you or your customers. Being registered for VAT would then allow you to reclaim the VAT you incur on related purchases.
As you haven’t started yet, I’d suggest that it would be worth discussing your plans with an accountant.