See page 156 onwards of
this document to see cancellation charges.
You need to check for which package they are on but for BT Broadband, line rental with weeekend calls it would be £20.50 per month so just over £180 for the 9 months left.
As others have said if you upgrade with BT then they don't normally charge but if you leave BT to go to another ISP then they will.
Edited by ian72 (Mon 30-Apr-18 08:17:15)