We often have family visiting from abroad (non UK residents with non UK drivers licences) and have lent them our car several times. All we do is call our insurance company and add a temporary additional driver for x number of days and it's no problem. It costs about £40 or £50 from memory but still cheaper and more convenient than hiring a car.
Is your FIL requesting you are added to his policy as a temporary additional driver? I know it is almost impossible to get an insurance policy in your name if you are non resident but should be no problem getting added to his policy.