I've been doing yoga for the past 6 years, and I seriously hate Bikram yoga. Like, big time. I live very close to the Balham studio, and bought the 10 classes for £10 introductory offer. I went once, and cursed all the way home.
I know some people enjoy it, so I wont go too far down the rabbit hole into my anti-Bikram yoga rant. I WOULD recommend trying Vinyasa yoga, which is challenging and gets your blood moving without needing to turn up the temperature to "fiery depths of hell".
The studio I go to is Surya Yoga near Clapham South, its a nice studio space with nice lighting and a nice peaceful atmosphere. Its also a lot quieter than going to a Bikram studio, which is often packed (I tried a class at 3pm and it was bodies to the walls full).