It's a breach of contract and you have a personal right against the seller since in effect, you have not been given good title to the goods. You can sue under normal contract law or, if the garage is a bona fide business, the sale of Goods Act 1979 - S.12(2)(a) - since the goods should be free of any encumbrances. The fact you cannot use the car as intended due to the failure to provide the V5 documents is an encumbrance on your title to the car despite the fact that otherwise, good title has been given.
How did you pay for the goods?
If you have a claim under the Sale of Goods Act (as it appears), and used your credit card to pay for the goods (the car), then the credit card company are joint and severally liable under S.75 of the Consumer Credit Act 1974. You can sue the seller or the credit card company or both.