Hi there
I also have a British passport and a SA ID Book and am back and forth regularly. I am a permanent resident of SA, not a citizen and it sounds like you are the same. You are worrying over nothing but I would advise you to stay over there where life is much better. UK is on a slippery slope and will carry on getting worse. I am hoping to get back to SA pretty soon and shall stay there for good where I can have a proper braai. I also worried the same as you are about coming out of the country, and it was totally not called for.
Cheers
Mike