Firstly, forex gains and losses are not subject to CGT or normal income tax if you are an individual and you are not a forex trader, i.e. buy and sell forex for profit. So if you deposit 100 pounds into your offshore account today valued at R2000 and then a few months later withdraw that same 100 pounds at R2200, you will not pay any tax on the R200 difference because it is not income, it just simply an exchange rate fluctuation. Note, this does not apply to juristic entities such as private companies, or natural persons who trade forex for profit who always have to account for realised and unrealised forex gains and losses in their income tax returns.
Secondly, no matter where you hold your offshore account, if you are a SA tax resident you WILL be liable for tax on any income you earn from those funds. So if you earn any interest, for example, you should declare this interest income on your SARS tax return. It doesn't matter whether or not you have repatriated the funds or kept it offshore, it makes no difference to SARS.