Your broadband is not true fibre but FTTC (Fibre to the cabinet) - your "phone line which is copper is actually acting as the final connection to the Internet, so it doesn't seem unreasonable that BT charge you rental for the line.HOWEVER, I agree that the level of the line rental set, is very high for someone not using the phone! And it certainly isn't "the law"!
BUT the "free" weekends and evenings package is not free (normally £3.15 per month) UNLESS you have agreed to a tie in of 12 months with BT. Be careful that the phone line contract is running concurrently with the Internet contract - there have been cases where people have had contracts for phone lines renewed before the internet contract ends.
Don't think you can do much for now - but negotiate like mad when your renewal date comes around!!!