It's a very straight-forward process. I converted my student visa to a marriage/settlement visa last year with no trouble whatsoever. Like peedal said, just fill out the FLR(M) form, collect a few pieces of information, pay the fee, and get your visa. I did it in person about a week after my wedding because I was feeling paranoid, but in retrospect it was so straight forward that I should have saved myself a few quid AND more than a few hours sitting around the Home Office and done it by mail. I will definitely be doing my ILR by mail!!!
Congratulations on your impending wedding!!!