Calculating profit needs simple mathematics.

Sales (Revenue) - Cost of Goods Sold (*) = Gross Profit (19% tax will be applied to gross profit, not to the revenue) 


(*) Your cost probably will be, Products cost, Your ads costs, Shopify costs, Payment fees and other expenses. 


If you want to learn more check this out: https://www.entrepreneur.com/article/226158

If you like to know more about the tax system in UK read this: https://www.gov.uk/government/publications/rates-and-allowances-corporation-tax/rates-and-allowances-corporation-tax