Buy a piece of utter trash for £250, garage it, insure it with Bell, get Drive Other Car 3rd Party.
Register the car you actually want to drive in your parents name and then voilla you can drive it third party.
Technically that is illegal but I know people who do it. One of my mates knows a guy who has been driving an Impreza WRX for 2 years since 19.
Insurance companies do nothing but exploit a legal requirement by intolerable fiscal greed, and premiums on younger drivers should be capped on lower insurance group cars as it's completely unacceptable. That or they should be legally required to refund half your quote for a claim-free year.