Originally Posted by hyperspace

AUP is not always guaranteed. You can find many threads around here where they dissect all the variables and probabilities of being offered an AUP. It's by no means guaranteed, even if the cabin you wish to upgrade to has empty seats. BA notoriously rather have empty seats than cannibalizing its premium seats sales by giving them "away" as sure and "cheap" upgrades at the airport.

Some says it's a combination of many factors, including:
how many times you took AUP in your flying history with BA
how long since the last time you did so
seats availability
pax status
pax CIV
etc etc

On a personal level, I almost always was offered an AUP when asked if available, but one time at LHR where they kept say "yes it's available", then typing some stuff and coming up with "oh no actually I'm sorry, I cannot do that".

Keep checking for award seats until the very last minute though, it's not unusual even on a busy route like LAX-LHR to see lots of last minute CW award seats been released 2-3 days before departure.