Once you have purchased the vehicle from auction/MOD disposals. You should be given a Disposal Form to enable you register your vehicle with the DVLA. On an age related registration plate. Of when the vehicle was manufactured (not the date it was released from the MOD).

If you are missing this form. A replacement Disposal Form can be obtained from DESA (Defence Equipment Sales Authority).

Email - DESLD-DESA-Commercial@mod.gov.uk

You will be required to send DESA your Vehicle History Card. And photos of your vehicle vin/frame number.

https://www.gov.uk/government/groups/defence-equipment-sales-authority 

https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/1091383/DESA_sales_brochure.pdf 

Once you have obtained a Disposal Form.

You can then proceed by completing a V55/4.

https://www.gov.uk/government/publications/apply-for-first-vehicle-tax-and-registration-of-a-new-motor-vehicle-v554 

https://assets.publishing.service.gov.uk/media/65705dfd746930000d4888d4/apply-for-first-vehicle-tax-and-registration-of-a-new-motor-vehicle-v55-4.pdf 

Lastly, submit both the Disposal Form and the V55/4 to DVLA.