Hi Juju,
I would really recommend Bliss Pilates just off Battersea Rise,
www.blisspilates.co.uk
their classes are small, no more than 4, and so the instructor can really tailor the class to suit you. Speaking as someone who completely screwed up her stomach muscles and pelvic floor by doing the wrong excercises post natally some years ago I would really advice not just jumping into a big class at first. After stomach surgery I went to see Shereen at Bliss, initially 1:1 then 1:3. She really helped me to understand my initial limitations and excercise safely. 6 months on I can do pretty much everything!
I believe she has recently taken on a childcare assistant so that those with children can attend her classes.