This is a very tough one indeed. Something that you may want to look into is a policy that insures you on another car....although only third party.
For example I think Cooperative insurance....may be called CIS...offer fully comp insurance with 3rd party cover on any other car. What is unusual about this is that they do it for the under 25's. When I was 18 I insured with them and it let me drive other cars, but only third party.
So how can this help you....if you bought a banger for £100, you could insure yourself fully comp on it for next to nothing [hopefully, perhaps anual mileage of 1,000]. If you had a policy like the above you could still drive the Zed third party and be accumulating your own NCD at far less cost than £6k pa.
Difficulty is that the Zed still needs its own policy - so perhaps you could get one of your parents to be the policy holder...guess they will have ncd and more years driving expereince and are technically less of a risk as they are older, so premium should be more reasonable.
If you went this route I am sure even the cost of running a near scrap car [tax, insurance, MOT etc] and parental Zed insurance would be way less than £6,000. But you need to be comfortable only driving the Zed 3rd party.
Good luck hunting, young driver insurance is nightmare!