Actually, Mikey if they closed, say, central London for three days (as they do in Monaco) and only allowed racing cars to take to the streets for a few hours the level of pollution would fall drastically.
I have to agree that the conversion of motorway hard shoulders to live lanes is a disaster waiting to happen. All it needs is a major pile some distance from a junction which needs the attendance of police, Fire & Rescue and paramedics and the outcome will be tragic. I was on the newly converted stretch of the M25 a week or so ago when I came across stationary traffic caused by a collision about half a mile up the road. The incident needed paramedics and I heard an ambulance and a police car bullying their way through the four stationary lanes for about fifteen minutes. They took another ten to reach the scene. Vehicles stuck in such circumstances simply have nowhere to move to. When I finally managed to pass the scene of the accident it seemed that no serious injuries had been sustained, but that was more luck than judgement. An F1 race around London's West End would present far less of a risk.