hi if you look on www.activebirthcentre.com/ you will find a facility for searching for local active birth yoga classes.
When I was pregnant with ds2 I went to Ruth Armes's classes in New Malden. She does them in the hall above the New Malden library and she is wonderful. After booking a place, you pay for a month each time (apart from the last month when you can pay as you go, as you don't really know how many weeks you will be around)! In addition to the yoga, there is always a chance to chat (or moan) about your experience of pregnancy, and although Ruth is only about the same age as me, she has three kids and has attended other births too, and seems to be very wise and is extremely helpful about many pregnancy niggles. Plus I'm sure it helped my body cope better with the demands of pregnancy, and getting through labour.
Ruth also set up the Birth Choice UK site which I think is great for allowing you to make informed choices about where to have your baby.
I also recently saw some ads for Active Birth classes in North Kingston but I can't remember the name of the teacher.
Hope that helps, anyway. Ruth really is a great teacher and I would recommend her to anyone - you don't have to be of hippy, lentil-weaving tendencies to appreciate her classes. She does 'birth preparation classes' too, which are probably better than the NCT ones, if you haven't already booked those.