Re: What's with all the Union Jacks in the trailer?
Well of course flying the Union Flag upside down is a distress signal. As for why it is being flown. Perhaps because that sequence is set in the UK, and the Union Flag is just a visual indicator that we are in the UK. Just as if it was set in the US the American Flag would be flying.
On the continent of wild endeavour in the mountains of solace and solitude there stood the citadel of the time lords, the oldest and most mighty race in the universe looking down on the galaxies below sworn never to interfere only to watch.