Not all terms of your employment are necessarily in your written contract. As you've had the van for 3 years, it could be considered an implied term of your contract that they provide the van IF your job requires the use of one.
Unilateral changes to contracts (changes made by an employer without the employee's agreement) can be justified on the grounds that they had some sound business reason for making the change. My advice would be to put your grievance i.e. the loss of the facility of a van, in writing. Any grievance must be dealt with under the statutory minimum Grievance Procedure rules so you will at least get a proper explanation. If you don't, you have grounds for a complaint to a tribunal.
You can stay in your job and still be compensated for the loss if you were to take the matter to an employment tribunal but you need to make your non-acceptance of the position clear from the outset. Saying nothing is usually deemed to be acceptance. Employees very often just keep quiet because they are worried they'll rock the boat. It depends how important this issue is to you. Do you have a union? Other people similarly affected? If so, speak to them and see what you can do on a softly softly approach.
PS I'm not a qualified solicitor but I have done a fair bit of employment law in my quest to become one so hope this helps.
[Edited on 6-10-2005 by Redruth]