within the UK, get onto ba.com / opodo.do.uk, get a price & then try any travel agency. or, if you still qualify, statravel or the likes.
anyway, here's a trick that works particularly well for uk/us flights:
1. grab a gbp 75 incl tax lhr/lgw-fra return ticket
2. buy a ticket departing from germany to final destination, sometimes even routing back through uk. www.opodo.de or www.traveloverland.de give a good indication.
did a quick search for a one week trip in feb mid week departures (all incl tax):
1. lon/cdg/jnb on airfrance: gbp 541
2. lon/fra gbp 77 on ba + fra/cdg/jnb on af gbp 429: total gbp 506.
granted, gbp 35 do not amount to tremenduous savings, but if you like flying and don't mind the extra 4 hours each way...it's like getting paid for the lon/fra flight.
also, for something different, there is www.airnamibia.com, which offers gbp 444 incl tax return from fra to jnb via windhoek, stopover possible.