Yes it's def worth going but can get very crowded if the weather is good. A good mixture of floats, vehicles, dancers, and costumes. Good for street people candid photography as well. You can visit St Pauls where there is usually a story about Dick Whittington and no problems about photography in the cathederal. A firework display from barges in the Thames completes the day. There is plenty going on at the South Bank but will be difficult to move once the firework display ends.
Good luck with the weather.