Bit surprised actually. It wasn't really about the trip to London. They don't go there until 40 minutes into the movie, and when they leave there's still over half an hour left. Instead the main focus is actually on the 4 graduating girls writing the song for Azusa that they sang in the graduation episode, and Azusa being worried because Yui's acting a bit odd.
That said, the part in London is the best part of the movie, and I was surprised that they got actual English people to do voices, which impressed me as I've seen plenty of anime with obviously Japanese people masquerading as native English speakers, which can be painful to my ears. (Looking at you, Beck)
If you liked the show, you'll love this movie. All your favorite characters are back, and just as likable as ever. The songs are just as impressive. It can be hilarious when it wants to, as well as make you cry at points.