KLM fly into MME daily, and have facilities there...
BA... do not.
MME being a small airport probably had only a handful of staff on dealing with their own KLM arrival plus the diversion KLM from NCL. A a full A321 wouldn't have got priority as the handling agent would be dealing with their own customer first. There fore diverting the BA back to LHR might have been the best idea. Even if BA diverted to MME, it might not have been able to go on anyway, you dont know how many hours the crew have done, the flight crew usually do multi sector days as do the cabin crew.