I can't comment on how much loading you're likely to experience with a U.S. license, as I've had a U.K. license since I was 17. However, to give some sort of perspective, my last renewal in July was around £180 for the year. That's on a 1987 Ford Bronco II 2.9L, TPF&T (third party liability, fire & theft cover), in a rural area which is in the lowest "risk band" for insurance, and with me as a 43-year-old driver with full no claim discount and clean driving record.
That's something of a specialist policy (I go via Adrian Flux as well), since many of the big insurers don't want to know you on an American vehicle, but certainly I know a couple of friends and neighbors who as similarly experienced drivers running 10- to 20-year-old run-of-the-mill vehicles for the U.K. pay similar rates - Maybe up to £300 a year if the car is worth a little more.