That is because it is a one off non refundable payment which by taking it out you get a 10% discount on the cost and are guaranteed no line rental price increase during the 12 months.
It is part of the Terms and Condition that you agreed to by taking the Line Saver that there would be no refund if you terminate the contract early.
It does not make it virtually impossible to change suppliers, all it does is mean that you have lost any remaining time left to run on the line rental.
Just a note that if you have paid line rental saver for the 12 months then you can not move your phone line from BT without losing your remaining credit. Which makes getting broadband with another supplier virtually impossible.
That's why you should always arrange that your 12 month line rental saver, and every other 12 month contract you agree with BT, all end on the same day.
Don't be tempted to sign up for anything that involves a minimum contract, even if it's free, on anything but that one day.
Looks like I may have been too trusting.
I was informed I would get a call within a few days to finalise and resolve my issues. No such call has been received.
Definately time to change provider.