SA flies from T2 as well.
If you are wheels down from AC858 (right?) at 1150, you'll be the gate at 1200 and in the immigration hall at 1215. Depending on how busy it is (T2 is pretty good, you can expect to be out of there by 1245 latest. You'll need to be back at T2 at say 1730/1800, giving you about five hours. Take your carryon with you, get on the Heathrow Express and you'll be in central London for 1330 by the time you've shifted on from Paddington station. Find a nice restaurant/pub for lunch, then have a walk around and get back on the HEX at 1700, recognising that it will be a busy time of day.