I don't work for BA but am a BALPA member elsewhere. I was also a TGWU (now UNITE) member in a past life, albeit under duress as I had no choice. There are some people on here who want all the protections and perks of Union membership but want other people to pay for it. I have zero sympathy for them frankly.
I think, however, it is ill-advised of BALPA and UNITE to actively ask their management not to give HP to non-members. It opens the door to a 'benevolent' act (divide and conquer?) from management to go against their request and give it out anyway. Furthermore, it is inconsistent in that the unions always negotiate pay rises for the whole workforce and not just their members. Finally, it has an ugly air about it that will only cause unnecessary offense. All-in-all, a very unwise move.