Interestingly Transport for London claims that the US embassy owes them ~£12 million in unpaid congestion charge fees, which exist as a disincentive to drive in central London. The embassy claim it's a form of taxation and as such they are exempt -- unsurprisingly TfL disagree. For context the next highest amounts supposedly owed are by Japan, Nigeria, Russia and India.

http://www.huffingtonpost.co.uk/entry/us-embassy-congestion_...