You've got plenty of good advice, and some stupid stuff too. This is not funny. So do all of these below:
Clock the registration, preferably video it, and report to Southwark Council as a complaint (polluting and wasting fuel).
Report to Southwark Council Environmental Health, and/or Highways, quoting their own air quality policies, and asking what action they will be taking. [www.southwark.gov.uk
Also question how on earth they can 'ban' unnecessary idling. Discourage it, take enforcement action yes. Ban? How many enforcement officers would that take? [www.southwark.gov.uk
Copy in the Mayor of London and ask what action he will be taking
Write to your MP and ask him to raise this with the Secretary of State for the Environment.
Copy in your local Councillor.
The stupidity of leaving your engine running is something I normally associate with the middle classes on the school run, or the police. Those have led to most of my arguments. The more we do to raise this with the appropriate authorities the better.
My only facetious comment is to stuff a potatoe in the exhaust so the vehicle stalls. Highy unlikely that this will send carbon monoxide into the vehicle.