If you only have a South African passport, you need a visa to visit the UK, it does not matter if you have Permanent residence in Germany. Your German Residence allows you to travel inside the Schengen area. There are no special rules if you are married to a German or other EU citizen. You still travel on your own passport.
The UK is not part of the EU, and is also not part of the Schengen area.
If you have a German passport, you can stay in the UK as a tourist for up to 6 months without a visa, but you must meet the standard visitor eligibility requirements.
Check if you need a visa
According to the Gov.uk website “You may be able to apply to come to the UK for free instead of paying for a visa if your partner or family member is a British citizen or from another European country.”
This means you still need a UK visa, but the Visa Fees may be waived if you are married to an EU spouse and you traveling with your spouse.
Please note, you are responsible for your own legal clarifications. No liability accepted.
Article 10 or 20 Residence card
From 1 January 2021 you can no longer use an Article 10 or 20 residence card issued by an EEA member state to travel to the UK if you’re accompanying your EEA citizen family member to the UK, or joining them in the UK.
An Article 10 residence card is a document that is issued under EU law (‘the Free Movement Directive’) by EEA member states to non-EEA family members of EEA citizens who are exercising free movement rights in a member state other than that of their nationality. For example, the non-EEA spouse of a French citizen who is living and working in Italy may be issued an Article 10 residence card by the Italian authorities.
An Article 10 residence card should feature the wording ‘Residence Card of a Family Member of a Union Citizen’. Another document, a ‘Permanent Residence Card of a Family Member of a Union Citizen may also be issued under Article 20 of the directive.