Tigs 2
Remember that the Armed Forces are exempt from large areas of UK and European Employment Law (yes the European placed exemptions for their(at the time legislation was being draft - conscript) armed forces too. The amendments are legal because they are Statutory Instruments - as are such items as the Working Time Regulations that apply to a wide range of the population. When the legislature changes things it doesn't need everyone else to agree. The legislature has given the Defence Council the authority to amend through Statutory Instruments - that have to be laid before parliament before they are enacted.
As for challenging, what are your options? Well not Employment Tribunals, Parliament has only given limited scope to hear cases from the Armed Forces (race discrimination, sexual discrimination and equal pay). That leaves the only course of challenge as a Judicial Revue (very expensive and you cannot get legal aid).
challenge it and you are guaranteed to win
The Gorilla is correct there. More have challenged and failed than have challenged and won. For some strange reason lawyers (including Mr Blades) tend to make a great deal of publicizing cases they win but are very quiet about the ones they lose.
One final thing, this is posted to illustrate the current position. Not to illustrate a personal opinion.