Eddie's point is not relevant - it's a benefit that you have been provided with and unless it was clearly tied to something writtewn down that said 'you get benefit X for doing Y' (namely car as a job need), then I reckon you are on strong grounds.
By all means use the grievance procedure, but just make sure you know where you want to go with this at the end of the line. If the grievance procedure fails (to your benefit), I believe you can take the matter to ACAS for a 'judgement' (not sure on this point - slowly getting out of touch for what ACAS can / cannot do), or you can resign and try and claim constructive dismissal (not recommending it - just pointing out options), or you can just lump it.
At the end of the day you have to take into account your long-term relationship with your employer, the value of kicking up a stink over this versus the potential to be seen as a perceived 'trouble-maker (not my belief, you understand just the view the management team might take), what your external prospects are.
In most of these situations, the company will get its wicked way over changes to T&Cs (mark my words and see what the outcome of the pathetic attempt by Unite to prevent BA) and it often boils down to the view taken by the company on the cost and/or the adverse publicity it may attract - externally or in the workplace.