BA doesn't need to have 100% of a constrained Heathrow in order to have a very strong (arguably dominant) market position. 55% will do quite nicely.
BA are finally waking up to the reality of how R3 would adversely affect their dominance of LHR. Conversely, with no R3, BA's market power will only increase, with an upward impact on yields. And yields are more important to BA's future profits than growth.
In theory the competition authorities should step in to prevent BA benefitting from an abuse of a dominant position. But perhaps BA is prepared to take this risk.