In my area you had to apply for the LAE supplement, and then, if your local manager agreed that you deserved it, you got it. In many cases this did not mean more money in your hand at the end of the month. Some actually recieved less due to increased pension contribution. I would get £18 a month if I accepted it before tax. I choose not to accept it until it suits me.
As for the union when has the LAE ever been backed by any of the BA unions even less cared about. Also any attempt to get another association or union recognised by BA has to date failed.