Ah, so you haven't started or ticketed, then I don't think that you have any recourse. A similar thing is happening to me. I wanted to go through LHR twice (one as a transit) which was allowed a month ago but this week I heard back from BA-MRU that it was no longer being approved by BA-RTW in the UK. In fact, given the difficulty they have in booking MRU-LHR as the first leg (which is the direction I want), they want me to start MRU-JNB-HKG.... and come back LHR-MRU.