Union representation for MF is clearly an issue that BASSA either can't or won't get it's head round.
When BA stated that they wanted a separate negotiating body for mixed fleet they were saying they wanted MF to be represented by MF crew who would negotiate terms and conditions for themselves, not existing WW CSD's who make up the majority of BASSA rep numbers.
If MF crew decide they want union representation there is nothing BA can legally do to stop them. All they need is sufficient numbers to join. They could choose to join BASSA or the PCCC or even the 'Magic New Fleet Happy Crew Smiley Club'. As long as enough of them joined the company would need to recognise the union (regardless of whether or not it has the word 'union' in it's title...).
Arguing the the whole point of MF is for it to be non-union crew really is a waste of time. UK law does not permit employers to do this.