Yes, it's legal and yes people have unsuccessfully contested it...
https://www.pensions-ombudsman.org.u...ads/p00960.doc
https://www.pensions-ombudsman.org.u...ads/p00290.doc
As for AFPS15, it actually works better than previous AFPSs as you accrue faster without significant career averaging (as most people stay within one rank during their time).
LJ