A really interesting experience that takes several hours to explore the entire vessel. The upper decks also offer some really unique views of the surrounding embankments including the tall skyscrapers in the City and The Shard. Once inside, if you visit the forward sleeping quarters, you can try out the bunkbeds. It's surprisingly hazardous, climbing ladders, squeezing in tight confined places (in the engine room in particular), so be prepared!
Getting tickets is easy, just show up, and save it for a sunny day for the best photos. Although even in wet weather, most of the experience is indoors anyway. By the way, if you collect Tesco clubcard points, you can exchange the points for free tickets online - which makes it an added bonus.