Contact the US embassy in London. I'm not sure, but I think the visa that you get at the airport, the stamp in your passport, is good for 90 days. You'll have to confirm that with them.
EDIT: You definitely should contact them to be sure, but looking it up, you should be good to just come on over.
From the State Department website:
Visa Waiver Program
Travelers coming to the U.S. for tourism or business for 90 days or less from qualified countries may be eligible to visit the U.S. without a visa if they meet the visa waiver program requirements. Select Visa Waiver Program to learn more, and find out if you meet the visa waiver requirements.
Currently, 35 countries participate in the Visa Waiver Program, as shown below:
United Kingdom
http://travel.state.gov/visa/temp/types/types_1262.html#2
EDIT: It looks like you'll also have to fill out the ESTA
https://esta.cbp.dhs.gov/esta/