Good analysis of LHR-CPT
here, including some yield figures. Given that SAA's yield on LHR-CPT is half the yield on CPT-PER, you can understand why they are not so keen on it. Note also (as pointed out here): they are continuing to serve the LHR-CPT market, just not non-stop - they will offer connecting flights via JNB. Given the (obvious) price-sensitivity of the market, that seems a reasonable way to go.
C.