I don't think that this is the case. There is no obligation in law, to recognise a new "union", where another union has collective bargaining rights, unless you can establish that the union in question does not hold a majority of members within the workplace.
You are totally correct - also the BASSA membership figure has to fall below 50% of the workers employed in the bargaining unit before BA can de-recognise BASSA as the trades union acting on behalf of Cabin Crew.
BA could also go down the route of Statutory derecognition of BASSA owing to lack of support for bargaining arrangements. They would have to make a request to a BASSA to end collective bargaining arrangements on the grounds that the union no longer has the support of the bargaining unit. The union can decline the request.
However, if the union declines a request after three years of statutory recognition, BA can apply to the Central Arbitration Committee (CAC) to hold a secret ballot.