Marriage, but not not intending to stay (for at least 1 year

I am an American citizen planning to marry my British fiance in Feb of 2012. Currently we both live in South Korea and we're getting married during our vacation, and then returning to Korea. My questions:

1. I know I need a visa just to get married (even though I'm not going to stay in the UK), but how long is the visa good for (when does it expire 3 months, 6 months, a year, etc...)

2. Can I simply mail my application to the processing center, or do I need to make an appointment to go personally.

Thanks for any help I can recieve.

1. Validity.
2. In person.