100% sure they are Brent Geese. I have made a Brent Geese compilation on my computer is you want to loook and compare. There are videos and lots of images, of a flock of about 200-300 Brent Geese.
Here is the link, I have put the video on YouTube:
YouTube - Brent Geese Compilation (With Music)
I have also made one without music, so you can hear the Brent Geese calls etc:
YouTube - Brent Geese Compilation (Without Music)