From an insider who used to work in insurance for a while. Your premium is made up from a combination of factors;
Firstly, how likely you to have an accident/ claim
So the following factors have an impact
Age, occupation, previous driving record, time of day that you drive (night time higher risk), tracker fitted, annual mileage, children and existing no claims bonus.
Secondly, in the event of an accident, the cost of repair/replacement.
So some marques are more expensive to repair than others. Also older cars are more expensive to repair than newer cars. Car value (likelihood of write off).
Thirdly, cost of payment of claim from another party.
So everyone will be subsidising the likely claim for the other party, inflated when bogus claims are submitted. Also everyone pays into a pot to cover uninsured lost.
Last factor, how price sensitive is the customer.
If you keep swapping providers you will often get a better quote. Loyalty is not often your friend in insurance, however companies will often try to retain you by providing multi car quotes as it is often perceived harder to leave. Equally you will get a better quote by applying a month or more before renewal. Leaving it to the last minute, or altering your quote too many times results in quotations.