You're right - it takes an Act of Parliament to close roads in England. The London Marathon and others manage it because it's for a short time only - a GP would need a week of closures.
There's another reason why London will never happen - racetracks shouldn't have roads with severe crowns. This is why there were so many accidents in Birmingham. Did anyone see the Q session there when Jean Alesi launched his F3000 through a large advertising hoarding ?
All London roads have strong crowns for drainage.