Aves,
Have you tried car comparison sites like GoCompare, Confused, Moneysupermarket and TescoCompare? If you haven't, then I suggest you try all of them because there are certain insurance companies which are not listed on some of them but listed on others. You will get a good idea by going to these 4 sites. If you still don't get a decent quote, then I suggest you try adding your parents (either mum or dad) as a named driver on your policy and see how that goes. If this still doesn't make much of a difference, then your only other option is to become a named driver yourself and have somebody else be the policyholder. The thing that you need to be careful with that though is by becoming a named driver, you are telling the insurance company that you'll be using the car not so frequently - the main driver will be using it more often than you and as long as you stick to that you will have no problems.
I suppose you've tried all other things like increasing your voluntary excess, putting less milleage for your car etc, haven't you?