Greyham - to give you an idea - here are the very approx tax levels. Please note that from what I recall of the rules, depending on the type of passs you get, you may be exempt from some bits of the US arrival taxes. e.g. ID75 versus FOC etc.

MCO to both FRA and LGW.....just under $20
FRA to JNB ... a bit over $105.
LGW to LHR by bus - price of ticket about $38.
LHR to JNB just under $170.

JNB to FRA or LHR.... about $52
FRA to MCO.... around $140
LHR to LGW by bus price of ticket about $38.
LGW to MCO ... around $146.

I note that VS and LH are the only airlines that fly into both MCO and JNB. Are you sure you can't get anything on SA or DL ?....they both fly non stop from USA to JNB, and you may be able to use someone else to JFK. DL do a handy MCO-ATL- JNB connection.

Oh and one last point - if you were to travel out of LHR or LGW in biz or first, the taxes would end up being even higher. (I assume unlikely to be applicable - but felt I had to mention).

Happy planning !!!

Lee