As previosly stated you can use whoever you want to arrange installation of a new line, you aren't forced to use BT retail.

Reason for the contract is in the past BT retail would install a line only for customers to immediately jump ship to providers who couldn't be bothered to arrange installation.

If you're that desperate for Sky and don't want the free BT offer then get them to install a new line - " If you are a new Sky TV customer taking Sky Talk Calls and Sky Talk Line Rental, we can arrange for a new phone line to be installed or an existing phone line to be activated (“New Phone Line”). We will confirm the charge (if any) for your New Phone Line at the time of booking. Sky will not install a New Phone Line where extensive new line construction over and above standard needs is required. "