When working for a company like BA every pilot needs to be in a union be it BALPA or something else.
I don't think I have ever been in a company where I feel so under attack. Profits come first, then shareholders and the wellbeing of front line crew is somewhere on the ocean floor. Sorry to say it but I remain in BALPA because of fear.
As for the BA/BMI hostilities you must be referring to CC because the only Axe I see is between the top and the bottom of the ex bmi pilot group