Hi,
Please refer to notice 741a regarding the place of supply of services.
Place of supply of services (VAT Notice 741A)
You need to determine whether your customer is a business or not for the purposes of receiving your supply and then look at section 6 gives the general rules and exceptions to this. Section 9.7 provides additional rules regarding work on goods which may override the general rule depending on if a private consumer is your customer. As you have advised the vehicles will be exported after your work is done please also refer to section 15 of notice 741a.
Thank you.