Can bailiffs take my car if it's only worth £400

Yes

If the car is the only goods available to the bailiff, he may take control of it regardless of its value.

If the car is under £1,350 and is used for somebody’s business trade, education or profession, then it is exempt from enforcement,^[1][2] and it is not bound under an enforcement power conferred against the owner.^[3]

^{[1] Regulation 4(1) of the Taking Control of Goods Regulations 2013
[3] See Making a claim to exempt goods
[3] Paragraph 4(1) of Schedule 12 of the Tribunals, Courts and Enforcement Act 2007}