Jonathan, it's the US, you know people don't walk, particularly politicians :-) Actually, that's unfair on our US visitor who does walk around London with us.
It reminds me of a friend's story. He and his brother walked two blocks to a bar. After they'd bought a round each, the barman refused to serve them a third round, explaining they'd be over the DUI limit. They then explained they'd walked from - for them - a nearby hotel. Apparently, the barman was astounded, as were some fellow customers; I believe they got bought several drinks for their amazing achievement!