Aygo/C1 etc. Although, I'm 20 and it's still £700 a year for my Aygo (had a reversing scrape in the Puma so +1 claim....). No car is cheap to insure at 17 because 1: insurance companies are ****s and 2: 90% of 17 year olds WILL understeer that car into a ditch and write it off within a week. Which is because of 3: driver education standards over here are shit.
Also, the Aygos are still worth a fair bit so that pushes the insurance up. And I'm willing to bet the Aygo's insurance prices will go up as the car's price decreases as Johnny Chav and his mates will be able to buy one at 17 and write them off. Conclusion, there is no cheap insurance for young drivers ever.