The payscales are reviewed periodically, but essentially they remain constant. They are uplifted with any payrises that happen along the way. Any changes to the payscales would be a much bigger issue and tend to happen when an issue has raised it's head to such an extent that either BA or BALPA (though normally BA) feel that the situation is such that it has to change.
Things are acute at the moment with regard to pilot numbers and the small amounts paid for overtime. There is much history to this but BA are desperate to weather the recruitment storm and not to increase the overtime rate. Traditionally BA run light on pilots and expect "martini men" to pick up the inevitable overtime. It is much cheaper to pay a little overtime than to employ another pilot(s). The problem for BA is that they pushed this too far and LH pilots are bouncing off the 900 hour limit and SH pilots don't pick up overtime as they are knackered, hence the recruitment problem. The upshot of all this is that overtime rates may change, but it is a race - will the manning this summer become so acute that to attract more people to do more overtime they have to make it more attractive? or will the recruitment panic deal with the problem so that the lack of bodies issue abates?
This is a typical reason to see a review in the pay system, though in this case not the pay scales.