Just to add to what Andrea said, you will not be able to apply for the visa prior to getting married and will have to supply the marriage certificate, either in person or via that post in order to obtain the visa. This means you do need to do a little planning.
If you are applying by post plan 3-4 weeks and DO NOT BOOK TRAVEL (normally takes about 10 days, but again, things can go wrong). If you plan to apply in person, the New York Consulate generally only releases appointments 2-3 weeks ahead of time, meaning that you may not get your target date far enough ahead of time. Again, you should not book onward travel, because there are people on this board who have booked travel and fallen into the small percentage that the processing isn't the same day.
Also, to probably answer your next question, there is no way to apply for the visa from the UK. She must apply for and obtain the visa in the country she resides in.