While I was working in London I took a black cab back to a hotel, I asked the driver where I could get a copy of the Taxi Driver's Highway Code. He replied it's hard to get hold of but there are only three pages in it anyway!!
IMHO, if someone doesn't want to wear their seatbeat, then thats their call as far as I'm concerned, as long as they don't stop their passengers wearing them.
And don't forget, if he has a smallish accident from which you emerge unscathed because of the seatbelt, but he's impaled on the steering wheel, you can leg it and not have to pay the fare.