Can bailiffs clamp my car whilst I am sitting in it parked on the highway?

No

Where a vehicle in use by any person at the time the bailiff seeks to take control of it, may not do so if it is likely to result in a breach of the peace.^[1]

"In use" means the vehicle is being operated by, the person.^[2]

^{[1] Regulation 10(2) of the Taking Control of Goods Regulations 2013
[2] Regulation 10(3) of the Taking Control of Goods Regulations 2013}