I agree with you. There are a few roads where I think restriction to 20 is overkill - wide roads round Sydenham which might even be A roads, for example, but the vast majority of 20 zones are so for good reason. I suspect the oddly-large 20 roads were included in the 20ing as a measure to reduce speeding-over-30 - enforcing all speed limits more tightly might slowly lead to a change in culture.