JT: "Is there a reason why South African (and El Al?) use terminal 1 at Heathrow and not Terminal3 or 2?"
Yes and it is the same as El Al. Security. In the days of the serious upheaval (19070s and 80s), SAA expected that their a/c would be targeted. By giving them the last stand on the pier, if an attack was made on the ground - they could easily isolate the end of the pier.
Now they have been there so long and their lounges and so forth - they do not want to go to T3 and that is full anyway!