You have to return to America for your fiance visa, no other way about it. What you do is get all the paper work you need to bring there while you are here. for what to bring.
http://www.ukvisas.gov.uk Then on that same site you make an appointment to the embassay in person once you have all the paperwork, if you go in person you will get ur visa the same day (in most cases) You cant go back to america then come back on a tourist visa because since you have been here for 5 months it will wave red flags at immigrations and you probably wont be aloud back in under as a tourist, Your best option is getting all the paperwork you need from him while you are in the uk, go back to America,make your appointment (which is usually abuot 2 weeks if not sooner) And it will be in New York,California (forgot what city
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) or Chicago, which ever is closer to ur American address is the one you have to apply to
Check out the visa stories section on here as well. It will help.
I was told that I have to be in america to apply, and that when it arrives I have to go to an embassy for an interview before I would get it to come back to the UK and THEN get married
Also, you dont get it before you go to the embassay, you make your appointment,go there for your interview and then if approved you get your visa
Legally you cant get married here on a tourist visa,they stopped that this past February. And please dont think about overstaying your tourist visa, if you do, you not only will be banned as a tourist for awhile,but it will also hurt ur chances of getting ur fiance visa.