This is not difficult.
1/ BA has a over-riding legal duty to protect its employees going about their legitimate instructed tasks.
2/ One employee group withdraws participation from the normal consultation process wrt to disciplinary processes.
3/ (2/) occurs against a background of some quite overt threats, and actual instances of acts agaisnt legitimately acting employees.
4/ In order to ensure 1/, BA institutes a process which still complies with all tenets of fair investigation, but expedites process (essential given the circumstances), and which ensures a consistent fair and equable treatment of all those subject to the process.
I have personally been involved as an employer in an ET where I refused access to witnesses by the accused or their representatives, and which involved bullying and intimidation. The Tribunal found that this was entirely consistent with a fair disciplinary investigation, in order to satisfy my duty & responsibility to protect employees going about legitimate business. Lots of ET's have done the same. And it is also pretty much the norm that given a serious gross mis-conduct accusation (as these are, whether BASSA thinks so or not), you immediately suspend the accused to get them out of the workplace and defuse the situation (I suppose that if an accused at the initial meeting came up with some overwhelming piece of evidence { eg it couldn't be me because I wasn't in CRC that day, I was in the Co hotel in Singapore, here's a copy of my roster}, that would then have obviated the handing over a pre-typed letter). The disciplinary can then proceed in a calm objective manner at an appropriate time. As long as the the accused are clearly informed about the nature of the accusation (perhaps with copies of witness statements), that is fair. It is not fair to put the bullied into a situation where face to face the accused or their rep can brow beat them and bully them into changing their evidence. Interestingly, in my case, the Union took the view that they were representing both the accused amd accusers, and therefore their only input was to provide advice to both parties in terms of process. It speaks volumes for BASSA and Unite in this, that they have seemingly unilaterally decided not to support those members who felt they were being bullied by their colleagues.