[Urgent]How can I get the USA visa if I am now in the UK?

若果你是在英國唸書或工作的，你可以到英國各地的美國領事館去申請旅行簽證。若你只是在英國旅遊，你就必須返回香港在香港的美國領事館辦理簽證手續。

http://www.usembassy.org.uk/cons_new/visa/niv/index.html
Processing Times The average visa processing time for successful applicants is five business days, during which time the applicant's passport must remain with the Embassy. Applicants who are advised at the visa interview that their application requires additional processing should allow at least 8 weeks for this stage of the application process. Once processing is completed, the applicant will be contacted for the passport. A further four weeks should be allowed for the visa to be issued and the passport returned.
http://www.usembassy.org.uk/cons_new/visa/niv/appointment.html

Appointments The service is available Monday through Friday, between 8.00 am and 9.00 pm, and Saturday, between 09.00 am and 5.00 pm, GMT (Monday through Friday, between 3.00 am and 4.00 pm, and Saturday, between 4.00 am and 12.00 am EST). The service is not available on American or British public holidays.

Callers from within the United Kingdom should dial 09042-450100. Calls to this line are charged at £1.20/min from BT landlines; some mobile and network providers may charge more. In addition, some mobile and network providers cannot access this number.

you can check with USA Consulate in England.