You could try arguing that the quality of car you have is an 'implied term' of your employment contract.
See here.
http://www.businessli...&r.s=m&type=RESOURCES
In this case it would be a 'custom and practice' scenario - you have enjoyed this quality of car for x years and now they are changing it.
Note though that an employer can change terms of an employee's contract - provided it does it through consultation - which should include explaining the reason for needing to make the change, and provided it gives you at least the contractual notice period (it sounds like it would be doing this as you say it applies at the end of the lease).
Even so, there may be some room to compromise on some salary compensation for the loss - you can but ask.