You can't be forced to work on your day off. It's against UK employment law AND most certainly against the EU Human Rights legislation. A collective bargaining agreement like BLR's can't overrule an individuals rights to UK legal protection.
I suggest that anybody who has been force drafted sends an Email to their FM to ask if force drafting is legal. Also a letter to BALPA asking the same question. It would be very interesting to see how BA/BALPA respond.