The principal driver of the car (irrespective of whether he's the registered keeper or not) must be the person whose name the insurance policy is taken out in. If someone else claims, falsely, that they're the main driver, they are committing a criminal offence known as 'fronting' (or, more formally, as 'fraud by false representation'). The maximum penalty is 10 years imprisonment (plus an unlimited fine):
http://www.statutelaw...2922461&filesize=2772
However it's perfectly legal for a young driver to add an older, more experienced driver (such as a parent), onto his own policy as a 'named driver. That often results in a substantial reduction in the premium.
Chris