If you read the info on the COA that Weby linked to, it says that if you intend to marry or form a civil partnership with a UKC and you are currently outside the UK, then you need to obtain prior entry clearance, ie either a fiance or a marriage visitor visa. Planning to get a COA before you enter the UK is not legal and could get you in trouble. What will you tell the immigration officer when you arrive at the airport? If you say you're coming to the UK to get married, he will want to see your visa. If you say you are just coming to visit your fiance, you will be lying, and IOs are trained to detect lies. You could end up being refused entry and, worst case scenario, being banned from the UK for ten years. Considering that you can't sponsor your fiance for a US visa, is it really worth the risk? Just get a visa, it's really not that complicated, and you will save yourself loads of headaches by doing things the right way.