Actually I think a more likely explanation suggests itself, having now seen the landscape version of the poster:
Belfast would have been right behind the text on this version and I think someone has knocked it out to make the "T" in "festival" a bit clearer. The same photo may then have been used on all versions of the poster.
It's still not an excuse because at best it shows total ignorance of London's historic cityscape on the part of the ad agency responsible.