So in [this] (https://youtu.be/rePb2NadxQw) clip (skip to 4:57) you can see it a little better in wide shots, and it looks to be the British flag, just furled up in such a way that the sharp 45 degree angles of the Scottish flag are not clearly viewable. Interestingly enough, the clip also points out something important many forget- Churchill was very soon after not PM at this time, Clement Atlee would be, but Churchill still held a strong position as war leader in the public's mind.