The £1,000,000 note at the time the film is set, 1903, would be worth approximately $100,000,000 ($100 millon) in 2002. Or £65,000,000 Sterling. (note to imdb. Source of calculation is www.eh.net)
The clues indicate that the time was after 1912 and before WWI. The American flag at the consulate has 48 stars, which became official on July 4, 1912. Also, there are very few automobiles, which although not entirely common, were around in 1912 London. By 1915 and 1916 automobiles were quite common. The costumes and other timeframe indicators suggest the time of the film must be mid to Autumn 1912. At this time, the actual exchange rate would have been one British Pound equaling $4.70. So a 1 million pound note would be worth about $4.7 million dollars. There are several sources to confirm early 20th century pound to dollar exchange rates.
The prop £1,000,000 note was larger in both size (about 7 x 9 inches) and value than any real note produced by the Bank of England up to that time, even notes for internal use. However, the bank still imposed strict regulations, which were violated when posters advertising the movie showed a reproduction of the note. This had to be covered over before the posters were allowed to be used.