Commercial vehicles are VAT qualifying so that VAT registered businesses can claim the VAT back on the amount that they depreciate over the time they own them.
When the first owner buys it, he claims back all of the VAT on the purchase price. When he sells it, he has to sell it plus VAT. He then has to include that VAT amount on his next VAT return, therefore pays that amount back to HMRC. And so it goes on, until a private individual buys it (you) and cannot claim the VAT back. As you will have paid all of the outstanding VAT, it is no longer VAT qualifying, even if you sell it to a business.