MaxAOB, your info is not quite right either.
It is a little more simple than 'having aggreements'.
If BA can't operate a flight due to a/c or crew going tech, they need to find another a/c and crew to operate the flight or cancel it. The prefered company to ask for BA to do so is GB for (like it or not) obvious reasons and visa versa. By the way, the two ops departments happen to get on very well and co operate whereever they can.
So, the BA aeroplane refered to earlier on in this thread was working on a 'sub' for GB.
GB often ask BA to operate a flight for them in times of nead but BA do not have much spare capacity so the next route of call is Excel, Air Atlanta...etc.
Regards to GB and BA pilots alike.