I agree it is a limited use of the RtW ticket, you could almost certainly fly for less as four separate return journeys from the UK if you simply choose the cheapest ticket, but of course that may not be what the OP wants to do.

With regard to 'where to safari', it really depends what the OP expects and wants from it. The landscapes, animal diversity, remoteness are all very different in different regions, as well as the costs. Also need to consider timings if visiting northern and southern hemisphere.