It's not just London. For example the small city I now live in (Lincoln) has very few, if any, roundabouts in the city itself - they are confined to the ring-road around the place, and roads in/out of it. Not true for all places, of course - for example Swindon is notorious for its "Magic Roundabout".
As someone that has lived in London for nearly 30 years, I can safely say - no they don't. Most intersections have traffic lights.