Strictly speaking, if you phone up, give them your situation you'll probably have to pay 2-3 months half price or something, then they'll cancel it. You're 'borrowing' and i'll use that term VERY loosely from the company themselves so they lose fuck all.. its all a con to get you to pay more.

Just ring up, give them your situation and they'll sort the rest out. done it with the AA and about to do it with my mums insurance.