Hi
Short term rates will apply making it more expensive to do it this way.Don't be tempted to arrange a policy by instalments and then walk away. Insurers will apply short-term rates and will bill you for any additional amount due. If you don't pay they will count this as a debt and will pursue you. Also by cutting short a policy year, it will only delay getting another years no claims discount, or claim free year if you have max NCD.
My understanding is that the 25 yrs age issue is not exactly true. Between the ages of 25 to 35, I believe Insurers gradually apply less of a premium loading due to age, but you should not expect premiums to suddenly reduce when you hit 25. Also alot of other risk factors are taken into account, i.e. claims, where you live, no. of years licence held etc, etc.
Hope this helps
Huckster