AZ is not saying that at all. The South Africans have halted the AZ roll out because they say in a trial of 2,000 people (with an average age of 31) the vaccine provided "minimal protection" against mild and moderate cases.
The original COVID in UK is still the most prevalent virus, the SA variant is circulating but not, it would appear, in huge numbers yet so the AZ vaccine will still work. Having said that no vaccine has 100% efficacy anyway.
Read this all the way through (not just the headlines) to find out what AZ and PHE have to say: https://www.bbc.co.uk/news/health-55921815