I could be wrong, but I seem to remember your BIK figure gets added to your other income to decide on your income tax rate.... so you don't just need to be earning under 40k and then have a car... you need to be earning under 40k MINUS BIK or you will tip into 40% for your BIK, with a massive double whammy.
The 40% band itself is a joke really - it used to be the preserve of the truly rich, these days its hardly that. I know its more than a minimum wage salary rate but its still hardly driving around in a ferrari territory.