Self employed courier

I have just started to act for a self employed courier. He is VAT registered but his turnover is below the statutorty registration limit. He rents a van on a week by week basis and claims this, and all other motor running costs, in his accounting records.

My question is whether he could claim mileage allowance against his profits rather than the actual expenses.

I am wondering whether  a) being VAT registered  or  b) it is van not a car   excludes him from claiming mileage ?