You don't need a transformer but you will need one or more plug adaptors. From some googling it seems that botswana uses a mixture of British 5A, 13A and 15A plug types.
Unfortunately in my googling I failed to find a source of BS546 to american adaptors I was happy to reccoment. The only ones I found had horrible universal sockets (the type that fits lots of plugs, none of them very well) and the BS1363 adaptor in the range was unfused which doesn't inspire confidence (BS1363 plugs should always be fused).