And here I am with an update...
Just got back from London after seeing Metallica at the O2 last night (**** yeah!!!

) and then headed to the SA Consulate on Whitehall first thing this morning...
We arrived just before opening time to a crazy huge queue, perhaps 40 people... No matter; we made some small talk and hung out with our queue-mates as we very slowly crawled our way along to the door. After a little under an hour of standing in the queue we were nearly there and a smiley man in a suit came along the queue and greeted us, we found him very friendly and reassuring; he quickly checked what we were there for and mentioned that we would need several copies of various documents and numerous passport photos - all of which my organised wife had with us, about 4 copies of everything you can imagine; birth certificates, marriage, drivers license, passport, etc. but it looked like it was going to work out. Ten minutes or so later and we were shuffled over to the counter to a very grumpy looking man...
Mrs. Rufus: "Hello, okay so I moved here when I was a minor and-"
Grumpy Man: "Wat are you here for?"
Mrs. Rufus: "Oh... well you see I have just gotten married and-"
Grumpy Man: "No... wat are you here for?"
Mrs. Rufus: "... To renew my SA passport, but it's a littl-"
Grumpy Man: "Give me the form"
He glances down the form momentarily then points to a statement on it, it's a bit about having a certificate which proves you are British National, but my wife was never given anything like this (or if she was she doesn't have it, as she was very young when she came here and her Mum passed away quite a few years ago now and we've no idea where she might have kept something like that):
Grumpy Man: "Do you have this certificate of nationality here?"
Mrs. Rufus: "No I don't, but you see it's beca-"
Grumpy Man: "You must apply to get this certificate from the UK Passport Office at this address"
Mrs. Rufus: "But see here I have my unabridged Birth Certificate which clearly shows that my Mother was born in the UK, giving my nationality by descent, and I have my UK Passpor-"
Grumpy Man: "A passport does not provide your citizenship, you must get the certificate"
He shoves the form along with a piece of paper giving the UK Passport Office address back through hatch to us and motions for us to move away... and that's that... The Smiley man also informed us that since my wife never had a SA i.d. book (as she was too young) she will likely have to apply for that first (taking ~6 months) and then apply for the passport (taking ~4 months). So basically we're done I think... with that long a lead time she might as well just do the renounciation (~12 months) and we'll just have to cancel our honeymoon, losing the ~£2k we've paid for flights/pre-booking in the process
Our only hope, and it is a small hope, is that
somehow the renounciation doesn't take as long as they say, as we can hold onto the booking without paying any extra until June next year and cancel it then... but it's not looking good