參見美國駐英使館公告(section 4): If you are physically present in the United Kingdom then you may choose to apply for a visa through the U.S. Embassy in London. You should note, however, that during your interview you can expect to be asked to demonstrate that your social, economic and familial ties outside the United States are sufficiently strong to compel your departure from the U.S. after a brief visit. Consular officers have no way of evaluating and verifying an applicant’s ties to another country. https://uk.usembassy.gov/visas/u-s-visa-and-travel-faqs/
If you are physically present in the United Kingdom then you may choose to apply for a visa through the U.S. Embassy in London. You should note, however, that during your interview you can expect to be asked to demonstrate that your social, economic and familial ties outside the United States are sufficiently strong to compel your departure from the U.S. after a brief visit. Consular officers have no way of evaluating and verifying an applicant’s ties to another country.
https://uk.usembassy.gov/visas/u-s-visa-and-travel-faqs/
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