Ok, the argument was the following:
Friend said that to get cheaper insurance, he would insure the car from his aunt's address (outer London, as opposed to central, approx. £400 cheaper), but would then have to change the address on his license to the same address.
I said that it was illegal anyway and that his insurance would be null and void, as obviously the car would be insured for Kent but be driven in London permanently, and that it didn't matter whether the license was the same as the insurance, but that the address the insurance were given had to be where the car was kept. He said that it was illegal to have a driving license in one address when your car was stored/insured from another.
He wouldn't believe me when I told him this was rubbish, as what about people with licenses but no car etc, but no, I had to get my point backed up before he'd accept I was right.
Thanks anyway!