You can discount your flight without using avios points due to a website glitch.
What you need to do:
Proceed to book flight. At the booking screen after selecting your flights you can select to use avios points for a discount.
Choose the maximum discount available with your number of avios points (125, 60 etc.) and proceed through booking to the payment page.
Once at the point of entering your card details, use the browser back button to head back to the initial page that you selected the avios discount on. Choose to remove the discount (Not Now / None)
DO NOT CHANGE ANYTHING ELSE. IGNORE THE PRICE DISPLAYED ON THIS SCREEN
Proceed forward to the final payment screen, and the price will still be discounted