You could apply for COA if you want to get married in the UK.

http://www.ind.homeoffice.gov.uk/6353/1 ... eapr07.pdf

 

It's not all that hard to prove a relationship: first relax and take a deep breath.

Second: as you've only known one another for 2 months, you are most definitely in the "honeymoon period" for the relationship - since you have another 6 months left to go before your visa expires, there's no need to rush into doing anything just yet.

A) Third: as you've only known one another for 2 months, you are probably not living together; however, you will be exchanging telephone calls, SMS, going and seeing places and taking photos with the two of you together, going on weekend breaks (keep the receipts), that kind of thing. That proves a relationship.

B) If you do happen to move in with your belle, make sure you declare to the UK electoral register that you've moved to that address: as a SA citizen, you're not eligible to vote, and the electoral commission will send a letter stating this. Additionally inform the local council you've moved there so that your name appears on council tax records. Those help to prove that you have "lived in a relationship like a marriage", though that part is really unnecessary.

C) It is also not the end of the world if you find yourself back in South Africa for 6 months, provided you make use of technology and keep the relationship going (and keep records of all of it): admissible proof of relationship includes emails, MSN messenger (or equivalent) logs, Skype call logs, postcards. The difficulty of making an international relationship work also lets you know the strength and calibre of it.

In principle there's no reason why you can't apply for your COA at the end of next month based on whatever you can put together for (A). There's nothing to stop you moving in with your belle in a couple of months and doing the things in (B) while waiting for the decision. It means that if the worst happens, and you find yourself back in South Africa for Christmas, your application for a fiancé's visa should be a breeze.