Different insurers and underwriters will look at different rating factors.
Firstly they will look at what kind of customer they want, they will determine what rating factors they will focus on to ensure that the type of client they require will be happy with the price then they market it at said individuals.
It also helps as this avoids having larger companies taking a monopoly of all drivers.
Saga's biggest rating factor is obviously age, they're synonamous with older customers. They feel that if they insure who they percieve to be 'safer older drivers' and refuse to cover boy racers they will reduce payouts, reduce premiums and increase revenue. I can also tell you that an insurer makes no profit on a policy holder until the 3rd year of custom.
Companies underwritten by UKI, the uk's 2nd largest underwriter.....also my employer, focus largely on the area where a vehicle is kept and we are updated regularly with crime reports and accident details. It is only recently they have taken to insuring in Northern Ireland again. If you live in the greater manchester district, Birmingham or surrounding areas of London you will pay a massive premium. If you live in the Hebrides of deepest, darkest north wales you will pay chump-change for your policy.
It really is best to ring around, also use the internet whenever possible as well.
I hope this answers your question.