if this was me I would read all of the terms and conditions to see what is written about leaving early. Who to request this cancellation from and what is the cost of ding the cancellation.
You can allways change to another provider as I do not think Sky or anyone else could stop you, but you may have to pay both for a period of time unless you know what is the correct cancellation process/term as per the first sentance above.
I thought there was a pocess where you paid a % of the remaining rental period, but the terms and conditions should be clear.
IanD