Other mistake: In the scene when the helicopters arrive over the research center, look at the heli in the bottom right - it just fades out, while one on the left materialises, and the one in the middle changes position. Notice that after all the helicopters have changed place, you can see the helicopter that faded out appear on the helipad. (02:25:25)
Continuity mistake: When the Senior Civil Servant is reading Dowding's letter there is a knock at his door. In the next frame the Civil Servant has removed his glasses, appears more leaned back in his chair, and the position of the letter/page has also changed. (00:06:00)