Round the world?

Hiya, Im considering doing the following trip. London - overland China,
(via Russia), fly Thailand, fly Australia, NZ, and back to London. Not sure
about homewards route, perhaps America? Would definatly like to fit in cook
islands, Fiji, or some other tropical place if possible.

The problem is I want to goto South Africa to do some sararis etc. Im not
sure if I can fit it in on my return route, as round the world tickets with
star alliance and one world, dont let u go back on yourself, (and I would
have to fly pass uk, if comming from nz).

I could do South Africa on outward route, but really want to do journey from
London - Russia/china overland.

Any suggestions please?

I am a low-cost student backpacker, so can only afford really cheap flights
etc.

Thanks Phil :-)