Try the Bravura Creative Art Studio, 612 Fulham Road - tel. 7731 7633. There's also one in Chiwick High Road perfect for kids round the Turnham Green area.
It's not in London, but in King's Lynn - a pottery run by two smashing chaps where you can have a paint a pot party for children [birthdays - an unusual and creative activity while parents can relax!] or adults [hen night, engagement, pre-dinner entertainment - bring your own wine]. Details on 01553 76 78 98
"Brush & Bisque-it" in Clapham (Northcote Rd) (also in Barnes)
and I'm not sure the following are still around, but were a year or two ago:
"Art4Fun" on Fortis Green Rd in Muswell Hill, and "Colour Me Mine" on Muswell Hill Broadway
"Bridgewater Pottery Cafe" on Fulham Rd
and "Art Attack" somewhere around Portobello Market
The Guild of Ceramics Studios keeps a list of all accredited pottery painting studios throughout the UK. There is one in nearly every major town. Go to countryloveceramics.com to find the locations and then call 01235861700 for contact details. Happy painting.