When getting quotes put the 17 year old in as the owner and main driver and get quote. Then repeat exact information and add a muture driver like mum or dad as a named driver on his policy.
That will bring the price down. Or do a multi car quote from admiral and put all the family cars on.
I think you will be pleased with your quote.
I was I saved £2000+ on multi car.
370z mine
Quashqi wife
Honda type R 20 year old son
Saab 95 aero HOT 22 year old son
Give it a go.