Self-employed? Or not...
One thing I see in many transport firms is a mixture of drivers who are 'on the books' and some drivers being self employed contractors. This is perfectly acceptable to meet operational requirements and cover busy periods by hiring in extra help.
However, what isn't acceptable (in the eyes of HMRC) is listing drivers who are essentially employed to drive exclusively for the company as self-employed and having a contractor relationship purely to reduce the company tax liability.
To be clear about the government's position, see the below link: