Hi i,m not in the uk any more ,but it worked like this in the past ,firstly only so much of the car was written off for tax ,as you may use it privately as well, unless you can prove it was for biz only ,like a sign written van,
secondly the tax is written off like over 3 yrs as a like wear and tear of that asset,
ps ask if you should go limited company route too ,as it gives you a lot of protection..hope that helps a little
