Kraut,
The problem with your theory of multiple pilot unions won't work with a British company.
As far as I'm aware the company only have to deal with one union that I assume has the highest membership amongst the pilot group. So unless the German pilots union can recruit sufficient numbers of pilots into it to outnumber BALPA, pilot representation will stay with BALPA.
On the strike front under British Law you would not be able to join a strike organised by BALPA if you weren't a member. Doing so could lead to disciplinaries.
United we stand, divided we fall.