BA membership of BALPA is less than 50% of the total and has been for some time now. If you think BALPA subs are high ask an American about their union dues!
BALPA is only as strong as the membership. Belong and elect people that will look after YOUR interests.
Interestingly that is what happened a year or so ago with the BA section. The BA membership was becoming increasingly disillusioned with their reps and much new blood was voted in. Much has now changed.
You have much to lose by being outside.