I got caught with this. The dealership gave me a letter to say I was taking car back to Ireland, but I got pulled over in north wales. The cop let me off as I was 45k from the boat and I showed him car was bought that day and fully insured. However he explained it is completely illegal to drive on any road in the UK without road tax and explained if he called it in, he would be ordered to impound the car. He said it was illegal for the garage to let me drive off in the cat untaxed also. They have cameras that scan plates on motorways and flag offenders, thats how I was caught. I guess you could stay off main roads and avoid them that way, or stick your own Irish plates on it to get it home but not sure if thats legal or not. Maybe get lucky, like me, and meet a sound copper.