I believe there was some restriction on operating to Tehran - something along the lines of 'the flag carrier' of the UK in this instance cannot operate. Hence why bmi operated it rather than BA. Might have got this story wrong though, I just remember reading it somewhere.
There was some concern that Tehran might have to be canned if BA couldn't get the traffic rights. Fortunately, common sense seems to have prevailed and BA have been allowed to operate there.