Vans come under the scope of Capital Allowances for businesses. You can therefore claim the cost of the van against your expenses, as part of the Annual Investment Allowance (AIA) that a business is able to offset against tax. (The limit on all investments for this is £50k - but that figure is unlikely to trouble you).
The gist is therefore that you can reduce your net income in the tax year by the whole of the cost of the van as part of your business expenses, and pay tax on the net figure of profit.
If you haven't had the van for the whole of the tax year, you would only get a proportion (proportioned based the number of days you had it, over 365 days) of the whole van cost in the tax year in which you bought it - you would then get the remainder of the cost as expenses relief in the following tax year.