Advertising is not a hidden agenda. It is a big business.
It is of course totally believable that it was an honest faux pas, done poorly but in good faith, and they then reacted appropriately to twitter responses by removing the post (note not the advert) quickly and issuing a statement. Obviously, that's the default.
I also find it equally believable that a company that spends at least couple of million pounds (based on previous years) on their biggest targeted advertising campaign for their biggest sales of the year, analyses every possible outcome of said camapign.
And that people whose job it is to do so, might have flagged that image, in the context of, if nothing else, daily public marches with those flags right across the UK. That everyone involved failed to notice it, I find borderline unbelievable.
I can believe that they would have assessed the risks, and been ok with issuing it on social media where it can be removed, while not featuring it in the actual video, accepting the limited fallout, and with it the consequential vast exposure.
That's just a take, the same as any other btw.