I had both of my sons insured on my Golf 140 GT diesel, cost £2400 (up from £220)for them as learner drivers but when they passed their tests the company refused to continue covering them. It took a day of wrangling for them to agree to it and cost another £600.
I bought a VW Polo 1.2 which is in my oldest son's name and put the youngest on as a named driver. It cost £1700 fully comp, that was last year when they were 17 and 18. It cost me a car but I saved about £1k on Golf insurance.
Oldest now has 1 year NCD and renewal was just over £600 for the pair of them.
In short I would recommend getting a smaller car, putting the insurance in your son's name and building at least one year NCD before getting a more powerful car.
You also have a chauffeur on call 24/7 and your protected NCD is safe.
PS I understand why premiums are high for young drivers but what I can't understand is why insurance companies charge so much to pay by instalment. I was lucky enough to be able to pay £1700 in a oner, If I had need to take the monthly option, which many young drivers will have to, many of the companies were charging almost double, a complete rip off.