Sorry to be a pedant but it will not be tax exempt. You will have to change the vehicle's class to "Historic Vehicle" and at the moment there is no charge for taxing a vehicle in that class.
Cheers
John
The GOV.UK website says that "You must apply for a vehicle tax exemption to stop paying vehicle tax. This is sometimes called putting a vehicle into the ‘historic tax class’."
You are not changing your vehicle to Historic Class.. You are applying for Tax Exemption, and if this is granted it is reflected in the change of taxation class to 'Historic' on the V5C
https://www.gov.uk/historic-vehicles/apply-for-vehicle-tax-exemption
