I think anything under £1500 is doing well for a 21 year old.
Beware of GeryAtrics suggestion, this is called fronting, the insruance companies are on to it.
If you get caught fronting, your insurnace could be voided
Besides that, with the insurance companies being on to it, you may find it’s actually more expensive.
You could try doing it the other way around. Find someone who’s around 50 years old, with maximum no claims, no convictions etc, and try adding them to your policy, you may find it comes down.
Mother and father for instance.
I know you won’t like my next suggestion, it comes with your age group… opt for a black box.
Doing these few things, I more than halved my daughters insurance to around £750.
Year two, the black box still offered a saving of around £300.
It wasn’t until about year three where the black box wasn’t worth having.