Sir,
If you get married, and file I-130 and I-485, for the adjustment of status, it is immaterial whether the visa, or more correctly, I-94, expires. It is possible that the U.K. fiancee comes here on a Visa Waiver Program, which is valid for 90 days. If not, it it is likely to be a B-2, valid for 180 days.
In short, you may get married, but the only thing is: the fiancee should not have come here with the intention to get married. Such presumption exists within 60 days of the entry.
This reply is in the nature of general information, not legal advice and should not be relied on as such.
Alex Berd, Esq.

