BA from LHR-CPT was very nice in First

Consider purchase with $$ ORD-LHR and upgrade (miles or whatever) and award to JNB or CPT on BA.

The issue you will have ORD-LHR-CPT (for example) is the morning LHR arrival with an evening LHR-CPT departure. You have the whole day to kill in London then another overnight flight.

Coming back the connection works fine .. not fine on the body as you are pretty tired anyhow!

The alternative would be to do a RTW trip of ORD-LHR-JNB (CPT) - HKG-LAX-ORD tho watch the CX flights as some do not have first ex-SA to HKG (business only).