I think the company car rules only come into play if you are an employee and provided with a car as part of your remuneration.
Thus the car is regarded as a perk and in effect is part of your salary, thus a notional value is attributed to the perk and you pay tax on it, as you would on the extra salary you would have if you didn't have a company car.
So at least if you're self-employed and use the car as your private car then you don't have to pay tax on it.
Of course, I'd prefer the big fat salary and the company car anyway
