Some are:

Car type
Car modifications
Kept Postcode
Garaging
Estimated annual mileage
Driving history
Driving convictions
Driving accident history
Marital Status
Number of children (you are more likely to be a safer driver if you may have children in the car)
Occupation (if you drove lorries for example, you are a higher risk due to tiredness when driving, however something like customer services is a low risk)
Age and driving history/convictions of named drivers (they can actually make your premium cheaper!)
The optional excess amount (the higher the excess, the lower the premium – but remember, you will need to pay this in the event of a claim so don’t make it more than you can afford!)

As for a 6 month policy, most insurers provide 12 month policies so cancelling it after 6 months may incur a cancellation fee. You may be better to go with a broker if you only want a short term policy.

Why not do a couple of online quotes? They’re free obviously, and you don’t have to take them up. You might find that the sites may tell you to call for a quote, but this doesn’t mean that it’s an automatic decline, it may be down to your age v the car you’re choosing. They may want to ensure you are a suitable risk before they confirm a quote to you.