Your best bet here is to contact CS after the flight and ask them to refund the price of the LHR-MAN flight. As it's not an EU/UK airline, there's no other way to force it, so the worst you can lose is a few minutes time.

I seriously doubt the EK17 flight was cancelled due to low loads. Everybody has been scrambling around trying to get flights back. As per the BA forum - all their flights sold out very quickly after the news.