Hi Docvenkat

You only need to book via the OnBusiness page if you are wanting to use OnBusiness points to book or upgrade. To buy a ticket with cash, upgrade to the next class with Avios (assuming there is reward availability in the next class) AND earn OnBusiness points, Avios and TPs for the cash element, simply login to your BAEC account and make the booking there (select "pay cash and upgrade with Avios"). You will be asked during the booking process if you have an OnBusiness number - alternatively you can go into your booking later via Manage My Booking and add it from the link right down at the footer of the MMB page.