If you're looking for a mum's class, there's Buggyfit and PowerPramming on Tooting Common. I think PowerPramming is becoming Trainhappy now.
For regular classes, I think OneElement, Swift, and British Military Fitness all hold classes on Tooting Common and there are probably even others. These all offer free trials too. Websites below.
I've tried all of the above except Swift. The buggy ones aren't quite as good of a work-out as the others obviously, but quite good socially to meet other mums.