So, will this happen all the time when the remaining MAN A330 goes tech?
In event of an AOG problem, bmi will arrange the fastest way of getting passengers to their destination and the aircraft back on schedule, same as with any other airline when they don't have any other aircraft available to cover an AOG problem. Suppose in this case using a United aircraft was expeditious, plus many on the flight would be United customers anyway. If it had happened this side of the Atlantic the likes of an Air Atlanta or EAAC B747 would probably be stepping in...
bmi obviously never planned to have a spare aircraft at Manchester to cover delays, the spare A330 was forced upon them due to it's early return from SAS and lack of other suitable work for the aircraft until now. It must have been a huge expense to the airline over the past year operating a very small long-haul network of two very new Trans-Atlantic services, under difficult economic conditions carrying the overhead costs of 3 aircraft.
The deal with South African is going to bring in a significant amount of cash to bmi which should help secure the future of both the Chicago and Washington services from Manchester. Over the winter bmi will now have better loads on the Chicago route with passengers being re-routed due to the cancellation of the Washington flight. Plus they also have no commercial risk in the operation of the other 2 A330s for SAA which will be in constant profitable operation, as opposed to one flying the loss making Washington route and the other sitting at Manchester for the 2 or 3 flights it would be required to make over the winter!
In the long term I think this makes sense.