LHR-JNB is codeshared with SAA, and as such BMI sells it through its website.

However, it may be due to the fact that SAA is not yet officially *A, as to why MAN-LHR-JNB can't be booked, as MAN-LHR can be added to other codeshared flights out of Heathrow.

As far as I am aware, another thing is that the site will only let your book flight with one connection, so no booking LHR-HOU via MAN and IAD. (I guess they think that no-one would be that desperate and not want to fly direct from LHR)

Personally, I think they should get rid of the dropdown boxes and replace them with input boxes like on Lufthansa's site, allowing the options of booking 'bmi only' or 'Star Alliance', and possibly a pop-up 'City Finder' like on Air Canada's site.