My son has been to Southwark gymnastics a few times and enjoys it, but he already attends lessons so knows some of the people which helps, for him anyway. You can book into their lessons in holidays which run 10-12, and add on pre or post lesson holiday club with other activities. The other one is Nimble Arts which run in Horniman and Heber schools. My son enjoyed that. Although not a frequent one as a bit pricey, but they do set up a lot of activities.