If they want the brand AND want to base out of the UK then, for all aircraft in excess of 100 seats, they must be flown by BA mainline flight crew. That was legally ratified during the Openskies dispute and even IAG couldn't get round it.
You've missed out the clause that this only applies to BA subsidiaries. IAG subsidiaries are not covered by the BA pilot scope agreement. Industrially, there is little to stop IAG setting up an airline and painting it in the same colours as BA aircraft. This is the whole reason why we've just recently accepted large annual cost savings to bring BMI into BA, rather than leave it as an IAG 'subsidiary' (if you could call it that).