It's illegal if you are the owner of the car and it's registered to you, but your dad is the main driver and you're a named driver (I think)
It's not illegal to be the main driver and add you parents though. That's what i did and it knocked £400+ off my quote. Still silly prices though, i struggled to get classic too as i'm only 23. I paid £345 for mine this year and £800 for the Abarth 500! (new one, not Classic)