Hi Roker,
1. What SKY mean with a 1 month termination of contract (even when you are out of contract) is the last day of the following month from which you gave notice.
2. However, as you are out of contract you are not liable for telephone calls made from the day you vacated. You need only pay for telephone calls made up to the date you vacated. You are liable for the extra month's standing charge though.
I don't support SKY's right to stick you with the "extra" month's liability. But, you are dealing with people who work in a call centre and who are replaced every few months. (I don't mean sacked, but they probably hate SKY as much as you).