When you click "Add Selected to Basket" after choosing the products you'd like to order you'll be taken to a page where you can review the order.
If your order does not hit the minimum order value there will be a red exclamation mark next to the Total.
From this page you can adjust the quantities of products already selected or add new products in order to reach the minimum order value.
Once this has been reached the exclamation mark will disappear.
If you ever need to change the minimum order value you can do this at a supplier level.
Please note: managers and admins can checkout orders which do not hit the minimum order value, but this should only be done if the supplier has confirmed they will accept the order to prevent any issues.