I would have thought that it would be plenty of time. I fairly frequently book the BA57 (normally planned to arrive 0905) connecting onto the 1040 departure to Cape Town, frequently missing the check-in deadline by 5-10 minutes due to immigration queues. The 1250 is what I am /always/ rebooked onto, you'd probably be fine booking the 1040 if it is all on one PNR. I have not done separate PNR down there for a while, though, so couldn't say either way on that one - if you were planning to do that.