You could point out to them that they cannot unilaterally change your contractual terms without your agreement, which is what they are trying to do. Whether or not car is used for business purposes is irrelevant. You could even get a lawyer to draft a quick letter for you to back you up on this (will cost a couple of hundred quid).
If everyone is getting similar cuts then you should club together in fighting this (use the union if you have one). If only you are having a perk taken away then you might want to add to your letter that it seems you are being unfairly discriminated against because this breach is causing your total benefits package to be reduced relative to everyone else's, which is clearly unfair and needs redress.
Pragmatically, though, you will need to weigh up whether taking a determined stance is worth it, or not. Will depend totally on situation (eg is this being driven by extreme business problems), how far removed your own boss is from decision-making process, and whether or not complaining might put you in the firing line for redundancy or similar.
Even if you complain you may find that they still go ahead with this. Large companies make minor changes to their t&cs all the time, and they may argue that this is a process of 'simplifying' terms. However, given that the effect for you is pretty major, normally they should compensate with salary uplift or similar, and this is what you should push for. There is a process by which companies can effectively force people to accept new terms by effectively dismissing everyone and then offering them new contractual terms (with potential redundancy for those who refuse), but it doesn't sound as if they've gone down this route.
There is not enough in your post to get full 'flavour' of what is happening here and why, but the legal situation is that no, they can't do this without your agreement. The issue you have is what to do about it, as there are consequences in complaining and consequences in accepting it.