Shirley Amatsakio is a great teacher. I think she's still running a class in Crouch End at the Haelan Centre. Check Owahan.com.
I highly recommend the Tai Chi & Qi Gong studio on Blackstock Road, Finsbury Park (www.wustyle.uk.com). They offer both daytime and evening classes and cater for Beginners, Intermediate & Advanced Students. Contact Studio Manager Faisal Mian on 07960 786772 for further details.