So wobble, BALPA gonna back you all the way are they just like openskies?
"Scope" is legally binding within the UK, as affirmed by BA under oath.
"Scope" requires that any aircraft certified with more than 100 seats (why do you think BA aren't using CF pilots to operate the 32-seat A318) is operated by mainline pilots. If CF do eventually end up with E190s, they will be crewed by (probably the existing CF) pilots on BA mainline contracts.
No doubt there will be some negotiation but the above is legally binding.