So net cost to the OP is the reissue fee + LHR fees + the WT+ "upgrade" payment + the cost of surface connection to Granada from some OW airport served from London (SVQ probably closest.) The benefits are (a) more comfortable flight JNB-LHR, (b) earlier qualifying for Plat hence double miles, and (c) maybe some lounge access provided a Plat card or facsimile can be obtained during the trip. Pretty simple benefit:cost calculation.

One other benefit might be the freeing up of an additional segment, in the event the original CPT-JNB flight was only to connect to IB. Otherwise the OP could take CPT-LHR and use the extra ticket somewhere where the additional miles and q-points would count for more, for instance JFK-SFO or JFK-LAX before connecting to SYD.

Actually, given that HEL and JFK (1) are just transit stops anyway, more q-points could be harvested by going VLC-xMAD-ARN-xLHR-xLAX-LIM.