Ok, if not giving reduced insurance premiums as a carrot for taking regular driver training/test, then use something else as a carrot. Tesco vouchers, discount flights or anything that might appeal to the masses.
Also wouldn't do any harm is we insisted on a UK driver training and test for anyone wanting to drive over here for more that a few weeks. Not just outside Europe and allowed to drive for a year before a UK test needed, but anyone who has not taken a test in this country and staying longer than for a holiday.