Admittedly, it was Westminster Catholic Cathedral, rather than the more famous Westminster Abbey, but the building was consecrated in 1910, so it's been there a while. In any case the Twitter feed for the party complained about a photo op in front of a mosque, which turned out to be the Cathedral. For the record, the Cathedral is built in a NeoByzantine style.

This incident has spawned the twitter hashtag: #ThingsThatAreNotMosques