Sky Broadband uses BT lines and equipment still as I had to pay to have a BT line installed in my new house before they would set the internet up, which means there's no problem leaving them
It's a 12 month contract and after that you can leave whenever you want to, although I've been very happy with the service over the last 6 months - way better than BT, half the price and twice as quick with the added bonus of a customer service/technical support service based in the UK