I don't know if you have had any responses directly from South Africans to this question, but here's what I know.
You should have no more difficulty on a South African passport throughout Africa than any other passport, and probably less.
The cheapest way to obtain visas, if you are not in a rush for them, is to go directly to the UK embassy / consulate of each country, get the paperwork and submit it.
There are other visa agencies besides the one you mentioned, but they may not be any cheaper!
The Visa Service, 2 Northdown St. Kings Cross London N1. Tele. No. 0171/8332709
Visa Express 31 Corsham St. London N1. Tele. No. 0171/251 4822.
And a few others at this
Good luck and let us know how you go!
"It matters not what goal you seek
Its secret here reposes:
You've got to dig from week to week
To get Results or Roses."
'One world, two wheels'
[This message has been edited by Susan (edited 15 December 2002).]