I do not know if other people often travel to other countries for just one show or exhibition but that is the case. :) Of coarse I have wanted to visit London before too but this exhibition is something I do not want to miss so it makes me finally put a date on that travel plan.
Another show I would really want to see is "Death becomes her: A Century of Mourning Attire" in The Metropolitan Museum of Art, which Trystan mentioned in a post too. Alas, the tickets from Finland to New York are way more expensive than tickets to London from that same starting point.
So, if you have some fun memories from London and tips where to go and what to see, I'd be really happy to read them in the comments!