The UK government is offering ongoing travel advice in the changing circumstances due to the Covid pandemic:
Following identification of a new variant of the virus, international travel from your area may be increasingly restricted because of domestic regulations. Different rules apply in England, Scotland, Wales and Northern Ireland. For example, those in Tier 4 areas in England will not be permitted to travel abroad apart from limited exceptions, such as work purposes. Follow all the rules that apply to you.
Other countries have closed borders, and may restrict movement or bring in new quarantine rules with little warning. Check our advice on things to consider, and be prepared to stay overseas longer than planned.
Before you return to the UK you must provide your journey and contact details. Also check if you need to self isolate.
Find more details here: https://www.gov.uk/foreign-travel-advice/south-africa