hallo, United States visa is not required for citizens of United Kingdom for a stay up to 90 days. it's true if you travel to the US on a tourist purpose. http://united-states.visahq.com/requirements/United_Kingdom/
You have to leave the country and file for marriage or leave the country and apply for a fiance visa, you cannot change the status of your visit while you are in the country.
1.Copy of the first and last page of your current valid passport. 2.One photograph as per specification 3.Printout of DS160 confirmation barcode 4.Original, valid HDFC Bank visa fee receipt. (Embassy...