As a Limited Company, you have the option to defer the tax on a van you've purchased by capitalizing the cost of the van and treating it as an asset on your company's balance sheet. This means that the cost of the van will be spread out over several years through annual depreciation expenses, rather than being fully expensed in the year of purchase.

The accounting transactions that would be required for this would include:

Debiting the company's cash or bank account for the cost of the van (£20,000)
Crediting a fixed asset account for the same amount (£20,000)
Recording annual depreciation expense for the van on the company's income statement, calculated using the straight-line method over the asset's useful life.
Please note that, from the information you've provided, I'm making the assumption that this is for a commercial use, please check that with your accountant. In addition, it's important to note that as per UK laws and regulations this is subject to change , please be sure to consult with a professional accountant to ensure you are following all applicable laws and regulations, and to determine the best course of action for your specific situation.