Can bailiffs clamp my car without giving a letter beforehand?

No

Bailiffs may not take control of goods unless the debtor has been given notice.^[1]

The letter is called a Notice of Enforcement, and it must be given to the debtor not less than 7 clear days before the enforcement agent takes control of the debtor’s goods.^[2]

If bailiffs clamped or removed your car without giving a notice of Enforcement, then there is a list of remedies available to you.^[3]

^{[1] Paragraph 7.1 of Schedule 12 of the Tribunals, Courts and Enforcement Act 2007
[2] Regulation 6(1) of the Taking Control of Goods Regulations 2013
[3] See Bailiffs took my car}