Please do not forget that BA does gains an advantage from the allowances in that they do not pay any pension contributions on allowances. The allowances monster started in the 70's when the then Labour government imposed pay freezes. Allowance were not included in this, so they were a way to pay staff more without breaking the pay freeze. For the crew receiving the allowance, the pay off for it being non-pensionable was the low, almost zero, tax paid upon this extra money. HMRC has finally recognised this money is mainly income and now taxes more of it.
Sadly this allowances system has since grown into a monster that is expensive to administer, and can cause wild variation in monthly pay. It has become very corrupt with some CC inexpicably getting lots of premium trips while others appear on these trips once in a blue moon.