Let's start with the "divorce certificate".
In order to be validly divorced in South Africa, one of you need to issue a summons for divorce against the other. That was probably your husband who did that. Then the Plaintiff (your husband) needs to appear in court and then the court will issue a divorce order. Once the divorce order is issued, then you are divorced. You do not have to have this order on your physical person at all and the validity of the divorce is not dependent on the order being registered at Home Affairs or whatever.
Now on to the second part of your question: In South Africa, you cannot conclude a valid second marriage if your first marriage has not been terminated correctly in terms of the Divorce Act. Since you got married in England with a Kenyan, South African law would not apply to your second marriage. You would have to find out from a UK lawyer whether your second marriage would be valid if your first marriage possibly still exist.
You can check your marital status (with reference to your first marriage) on the website of the Department of Home Affairs, alternatively, you can contact them through e-mail or telephone to check on your marital status in South Africa.
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