It's not uncommon to see BA use a subbed 767, it's happened many times before, and you can't really comment accurately without all of the correct information.
I suspect BA will have agreements in place with the airlines they plan to use as sub-contractors, especially when it comes to cost. They'll know the cost of running the flight and the revenue it'll generate, then they'll decided to run it or cancel.
Somebody might have made a loss on it... Titan? given all of the other subby work they do for BA a small hit worth taking. BA? upsetting then losing those passengers versus their future spend, a small hit worth taking. Who knows?
The people running these airlines know what they're doing, even more so than the would-be airline executives and operations officers gracing this forum.