http://www.google.com/finance?q=LON:BP
http://www.google.com/finance?q=bp<p>The pound is up 1%, which explains part of it, but doesn't come close to explaining the entire difference.
LON.BP * 187.87M = NYSE.BP * 3.13B * $/£<p>The LHS represent the market cap of BP at the LSE, the RHS the market of BP at the NYSE. Both sides should be approximately equal because 40% of BP shares are listed in London and 39% in New York.