I came over on a fiancee visa, we got married on a Tuesday, applied for FLR(M) on the Wednesday. Not an eye was bat. If you meet the requirements and submit the evidence, you get your visa.
You do need the certificate. We asked before we planned everything and knew we could get our certificate same-day. We also ordered several additional copies, just in case UKVI kept ours. The copies were posted to us later, but we had our original hand-written one for the application.