Can a car I am registered but not legally owned be clamped?

No

Bailiffs may take control of goods only if they are goods of the debtor.^[1]

Being the registered keeper of a vehicle does not make them the legal owner. The V5 keeper document even states the document is not a proof of ownership.

If a bailiff clamps a car not belonging to the debtor, the owner can make a third-party claim to controlled goods.^[2][3]

^{[1] Paragraph 10 of Schedule 12 of the Tribunals, Courts and Enforcement Act 200
[2] Civil Procedure Rule 85.4
[3] See Making a third-party claim to controlled goods.}