Moving house, I've had a plusnet account for 3 years and because my new address can't provide fibre (I was told 2 - 6 mb max speed) I've cancelled my account. After 20 minutes waiting for customer service I've been told I will be charged £200 "oh and by the way if you don't pay within 2 weeks we will refer the amount to a debt collection agency".
Alternatively had you renewed your contract and in an existing term still? I think you usually get charged to buy out your contract if you don't stay when you move. This is pretty common with providers who do big discounts up front as they don't make money in the free period so if cancelled in term they would lose money etc.