I think you should seek advice from the CAB as a second opinion. If you look at the attached link, it gives a pretty good set of information about the law on unfair dismissal.
http://www.bbc.co.uk/consumer/guides_to/employ ment_unfairdismissal.shtml
The eligibility of this is a minimum of 12 months continuous service (which you have). The case you would have to show is that your fringe benefits have been reduced without your agreement. Unfortunately to demonstrate this you have to resign and then claim 'constructive dismissal' at an Employment Tribunal and this may be a step too far for for you. However if you constructed a letter stating that you consider your employer is in breach of contract by this action and you request reinstatement of the previous arrangement, otherwise "you will have to consider your options". The mere threat of further action by you may force them to back down. The CAB can probably help you construct such a letter.