All depends who you're with. A lot of brokers will mug you right off and do it on a staggered monthly basis IE get to 9 months and you'll still have to pay for the full 12, or if you don't have to pay for the full term you'll still have an admin charge to cancel the policy (£75 with Budget insurance for example).
However you go with a quality insurance company (I'm with one), this will normally only apply when insuring direct and not though a broker, then its pro-rata what you pay, ie only have 6 months cover and you only pay for 6 months cover, and only £41 admin fee on top which is acceptable imo
Brokers can be good for specialist or cheap annual insurance as long as you never want to make any changes or cancel mid term.