To be honest, you're more or less wasting your time to write to the American Embassy in London. You can find out the same information by checking their website Visa Services U.S. Embassy London - in fact what they are likely to send you in reply is simply the printed up version of what is on their website.

If you're only looking to go to America for your gap year (i.e. not permanently), don't waste your time trying for a green card. You'll do better (and get faster results) if you can find an exchange program like CIEE (CIEE - a world of opportunity ) or something similar. That way, the exchange program will help you get the necessary visa and may be able to help you with the job search.
Cheers,
Bev