Except that people absolutely take that left turn despite the sign - it's just a very convenient shortcut, and UK has extremely lax enforcement on the road. The chances of getting a ticket for taking an illegal turn are close to zero. So Google will have the data that a left turn is indeed possible there, because clearly - people are taking it. Me reporting the "no left turn" sign is probably treated as malicious since clearly - in real life people are taking this turn all the time.