Maybe so Al however I would suggest where there is a will there is a way, any robust safeguard can be circumvented, just depends on how good your lawyer is. No doubt they have worked in the UK in the past is one of many justifications they have/will give to HM Immigration Services.
BALPA may not be able to stop P2F directly, however they can mobilise their members or at the very least release a press release against the practice which would no doubt be picked up by flight international and the national press. So far I have seen evidence of neither despite universal hatred of this.
What I am certain of is Jim McAuslan has shared more games of golf and 3 course lunches courtesy of "The Log" sponsor and P2F trailblazers OAA, than he has composed letters telling them he is unhappy with this trend they are promoting.