Yes, it will impact - you won't qualify for a fiancé visa if you will not be marrying in the UK... Because you need to provide evidence that you are planning a wedding in the UK in order to get the fiancé visa.
Also if you were able to get a fiancé visa, when you leave the UK to get married, you likely won't be able to get back in again after the wedding because you will no longer be a fiancé and therefore will not meet the requirements to enter again on your fiancé visa.
Plus, how are you going to explain your foreign marriage certificate when you are switching from a fiancé visa to FLR(M) after your wedding... When the purpose of the fiancé visa is to marry in the UK and therefore have a UK marriage certificate.
You have a couple of options:
1) apply for a fiancé visa, marry legally in the UK (at a registry office), apply for FLR(M) visa, wait for it to be granted, and then have a celebration in South Africa (you can't leave the UK between the wedding and getting FLR(M) so you would have to wait to go to SA).
2) come to the UK as a visitor for a few months (if you want to be on the safe side, you can apply for a visitor visa in advance for about £85), then fly to South Africa to get married, then fly back to the US to apply for a spousal visa... And finally, move to the UK
3) don't come to the UK at all before the wedding, just fly to South Africa to get married, then fly back to the US to apply for a spousal visa and move to the UK
4) get married legally in the US, apply for a spousal visa, fly to SA for the wedding celebration and move to the UK
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