the answer is the Union negotiating new, increased rates.
They are voluntary-if you don't like the rate don't do them-simple.
If an increased rate were to be negotiated then this will inevitably come out of the overall "pot" of money available and would then have an impact on every member of staff.
Surely it is sensible to keep the rate as it is in order for more to be available for any pensionable rise that may be offered?
Why should someone who has never done an AAVA-and there are plenty of them, potentially suffer in order to increase the rate for those who do?