I have booked using Avios in departures area of T5 (using my own laptop and internet tethering to iPhone) minutes before then using the iPhone app to generate the boarding pass and then going through security...^

In airports where they let you pass through security without a boarding pass such as the domestic terminal in MEL I even made the booking in the lounge, got the (delightful) dragons to print me a boarding pass and then wandered down to the gate...

Wonderful flexibility!