BA are now allowing holders of Avios to use them for cash discounts at the time of booking. The Avios need to be in your BA account (rather than your Iberia or AirMiles/Avious accounts), and you can select the number you want to use to save up to £30 on your ticket. (£10 is 1,500 Avios, £20 is 3,000 and £30 off is 4,500).
What I like about this is that you can see how many new Avios the booking will earn, and how many you have in your personal and family accounts. This is an example from a recent booking: