Fiance Visa -FLR Questions

Hello all. I'm an American marrying a nice English girl and we're going to live in the U.K. A couple of questions:

Once we're married, on average how long does it take to convert a Fiance Visa to an FLR by mail?

If I want to do the switchover in person where do I have to go? Is the only place in London?

Besides a marriage certificate and my passport, what other documents does the HO need? Do they need all of the documents required in the Fiance visa application?

Many thanks for all your help.
Patrick